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How does chemistry affect the clothing that we wear? Chemistry research is often full of surprises. One such surprise came to Stephanie Kwolek of the DuPont chemical company when she was working on materials known as polymers. These chemicals had been around for a while and were being used for new types of textiles. Kwolek was looking for a strong and rigid petroleum product. She came up with a material that did not look like your average polymer; she played a hunch and had it made into threads. This new material had stiffness about nine times that of any known polymers of the time. Further research and development led to the production of Kevlar, a material now widely used in body armor. Kevlar also found a range of application in racing sails, car tires, brakes, and fire-resistant clothing worn by firefighters. Materials Electronics Chemists are involved in the design and production of new materials. Some of the materials that chemists have helped discover or develop in recent years include polymers, ceramics, adhesives, coatings, and liquid crystals. Liquid crystals are used in electronic displays, as in watches and calculators. The silicon-based computer chip has revolutionized modern society, and chemists have played a key role in the design and continued improvement of these chips. The calculator shown below uses both a liquid crystal display and chips inside the device. Superconductors Many chemists are currently working in the field of superconductivity. Superconductors are materials that are able to conduct electricity with $100\%$ efficiency, meaning that no energy is lost during the electrical transmission, as happens with conventional conduction materials like copper cable. The challenge is to design materials that can act as superconductors at normal temperatures, as opposed to only being able to superconduct at very low temperatures. Clothing The fibers that compose the materials for our clothing are either natural or human-made. Silk and cotton are examples of natural fibers. Silk is produced by the silkworm and cotton is grown as a plant. Human-made fabrics include nylon, orlon, and a number of other polymers. These materials are made from hydrocarbons found in petroleum products. Synthetic polymers are also used in shoes, rain gear, and camping items. Synthetic fabrics tend to be lighter than the natural ones, and can be treated to be more water-resistant and durable. Materials originally developed as textiles are finding a wide variety of other uses. Nylon is found in a number of plastic utensils. Taking advantage of its strength and light weight, nylon is a component of ropes, fishing nets, tents, and parachutes. Summary • Chemists produce materials for electronics, superconductors, textiles, and other applications. Review Use the link below to answer the following questions: 1. Who developed Kevlar? 2. Where are liquid crystals used? 3. What is a superconductor? 4. What are synthetic polymers made from? 1.10: The Environment What are the effects of waste within our environment? Where to dump the garbage? Where to put the waste materials we don’t want, after we have made what we do want? For many years we just dumped the waste in the nearest lake, river, empty spot of ground, and didn’t worry about it. However, as we learn more about the effects of chemicals on living systems, we have become more concerned about the effects of all this dumping. In many cases, we just didn’t know what the long-term effects would be. Today, dumping is generally illegal and offenders pay heavy penalties of fines and jail sentences. Environment Many chemicals that were once commonly used were later discovered to be harmful to the environment, to human health, or to both. The element lead was once a common additive to gasoline and to paint. Plumbing pipes were once typically made of lead. Only since the 1970s has the danger of lead become apparent. It causes brain damage; small children (who might chew on objects painted with lead-based paint) are particularly susceptible. The use of lead in gasoline, paint, and plumbing pipes is now banned; new materials are being developed to replace hazardous lead components. Lead is still widely used in car batteries. In recent years, battery recycling has become common—about $98\%$ of car batteries are recycled today. The used batteries go to a processing plant where they are crushed and placed in a tank. The lead sinks to the bottom, to be separated and melted. The plastic can then be used in a number of applications. The battery acids are treated with chemicals to neutralize them before they are disposed of. How Chemists Help the Environment Chemists are involved in all aspects of environmental protection. In the case of lead, chemists measured the amount of lead in soil, paint, plumbing, blood, and other materials. Chemists studied the chemical processes in the body to see how lead did its damage. Methods for removal of lead from the body were developed with the help of chemists. New gasoline products were developed that fostered fuel efficiency and smooth running of engines, without the presence of lead. Chemists continue to look for threats to our health and the environment—and to search for alternatives—so that harmful chemicals can be replaced with others that will do the job as effectively, but without the harm. Summary • Environmental contamination is a major problem today. • Lead contamination has serious harmful effects on the body. • Chemists contribute in many ways to helping with issues of lead contamination. Review 1. Where was lead found in products? 2. What is a hazard of lead? 3. What happens to the lead in used car batteries? 4. List two ways chemists help with issues related to lead contamination.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/01%3A_Introduction_to_Chemistry/1.09%3A_Materials.txt
What can be accomplished through team work? Most jobs to day involve a fair amount of meetings and team involvement. It doesn’t matter what you do, you will spend a certain amount of time meeting with other people, sharing what you have done, planning projects, and organizing the work. Hardly anybody works by him or herself these days – everybody is part of a larger group. The Nature of Science Today’s scientists rarely work alone. Rather, most scientists collaborate with one another as part of a group effort, no matter the setting. The majority of research scientists work either for a company such as DuPont Chemical Company in Wilmington, Delaware or for one of many universities, such as the California Institute of Technology. Working as part of a group has many advantages. Most scientific problems are so complex and time-consuming, that one person could not hope to address all of the issues by himself or herself. Instead, different members of a research group are each tasked with a particular small aspect of a larger research problem. Collaboration between members of the group is frequent. This occurs informally in the laboratory on an everyday basis. Research groups typically have regular meetings where one or more members of the group may give a presentation to the others on the status of the research that they are doing. Progress normally occurs in small steps rather than grand, sweeping discoveries, and that progress is helped along by the teamwork that comes from working as part of a group. Modern scientific research is usually expensive. Lab equipment, chemicals, research space, and the upkeep of technical instrumentation all costs money. So research groups need to raise money in order to continue their research. Much of that money comes from government sources, such as the National Science Foundation or the National Institute of Health, especially in the case of research being done at universities. Private companies can fund their own research, but may also seek outside funding as well. Scientists write grants explaining the goals of their research along with projected costs, and funding agencies make decisions on which research projects they would like to fund. The long-term viability of most research labs depends on the ability to get and maintain funding. Communicating Results Suppose that your research is a success. What now? Scientists communicate their results to one another and to the public at large in several ways. One is to publish their research findings in one of many publications called scientific journals. There are many hundreds of scientific journals covering every field of science imaginable. In chemistry, there is the “Journal of the American Chemical Society,” the “Journal of Physical Chemistry,” and the “Inorganic Chemistry,” to name just a few. Some journals have a very narrow scope while others publish articles from many different sciences and appeal to a wider audience. Examples of the latter include the journals “Science” and “Nature.” Journal articles are often very complex and detailed. They must be accurate, since the research field as a whole uses these journal articles as a way to make scientific progress. Therefore, journal articles are only published after having been extensively reviewed by other professional scientists in the same field. Reviewers have the power to make suggestions about the research or possibly question the validity of the author’s conclusions. Only when the reviewers are satisfied that the research is correct, will the journal publish the article. In this way, all scientists can trust the research findings that they read about in journals. Scientists also communicate with one another by presenting their findings at international conferences. Some scientists are chosen to give a lecture at a conference, typically about research that has already been published. Many other scientists at the same conference will present their work at poster sessions. These poster sessions are more informal and may often represent research that is still in progress. Science Friday A Shot in the Dark: Alternative Uses for Squid Ink Squid ink is commonly thought of as a defensive mechanism used by squids, but could there be other uses? In this video by Science Friday, Dr. Stephanie Bush talks about possible other functions of squid ink. Snowflake Safari Next snowstorm, grab a magnifying glass and look carefully at snowflakes. Bullet rosettes, stellar plates and capped columns are just a few of the many varieties of snow crystals. In this video by Science Friday, physicist Kenneth Libbrecht shares secrets about snowflakes. Summary • Scientific research today is a team effort. • Support for research usually comes from grants. • Findings are shared in meetings, conferences, and scientific publications. Review 1. How is most scientific research carried out today? 2. How is research usually supported? 3. How do scientists share their findings?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/01%3A_Introduction_to_Chemistry/1.11%3A_Research.txt
How can we use problem solving in our everyday routines? One day you wake up and realize your clock radio did not turn on to get you out of bed. You are puzzled, so you decide to find out what happened. You list three possible explanations: 1. There was a power failure and your radio cannot turn on. 2. Your little sister turned it off as a joke. 3. You did not set the alarm last night. Upon investigation, you find that the clock is on, so there is no power failure. Your little sister was spending the night with a friend and could not have turned the alarm off. You notice that the alarm is not set—your forgetfulness made you late. You have used the scientific method to answer a question. Scientific Problem Solving Humans have always wondered about the world around them. One of the questions of interest was (and still is): what is this world made of? Chemistry has been defined in various ways as the study of matter. What matter consists of has been a source of debate over the centuries. One of the key areas for this debate in the Western world was Greek philosophy. The basic approach of the Greek philosophers was to discuss and debate the questions they had about the world. There was no gathering of information to speak of, just talking. As a result, several ideas about matter were put forth, but never resolved. The first philosopher to carry out the gathering of data was Aristotle (384-322 B.C.). He recorded many observations on the weather, on plant and animal life and behavior, on physical motions, and a number of other topics. Aristotle could probably be considered the first "real" scientist, because he made systematic observations of nature and tried to understand what he was seeing. Inductive and Deductive Reasoning Two approaches to logical thinking developed over the centuries. These two methods are inductive reasoning and deductive reasoning. Inductive reasoning involves getting a collection of specific examples and drawing a general conclusion from them. Deductive reasoning takes a general principle and then draws a specific conclusion from the general concept. Both are used in the development of scientific ideas. Inductive reasoning first involves the collection of data: "If I add sodium metal to water, I observe a very violent reaction. Every time I repeat the process, I see the same thing happen." A general conclusion is drawn from these observations: the addition of sodium to water results in a violent reaction. In deductive reasoning, a specific prediction is made based on a general principle. One general principle is that acids turn blue litmus paper red. Using the deductive reasoning process, one might predict: "If I have a bottle of liquid labeled 'acid', I expect the litmus paper to turn red when I immerse it in the liquid." The Idea of the Experiment Inductive reasoning is at the heart of what is now called the "scientific method." In European culture, this approach was developed mainly by Francis Bacon (1561-1626), a British scholar. He advocated the use of inductive reasoning in every area of life, not just science. The scientific method, as developed by Bacon and others, involves several steps: 1. Ask a question - identify the problem to be considered. 2. Make observations - gather data that pertains to the question. 3. Propose an explanation (a hypothesis) for the observations. 4. Make new observations to test the hypothesis further. Note that this should not be considered a "cookbook" for scientific research. Scientists do not sit down with their daily "to do" list and write down these steps. The steps may not necessarily be followed in order. But this does provide a general idea of how scientific research is usually done. When a hypothesis is confirmed repeatedly, it eventually becomes a theory—a general principle that is offered to explain natural phenomena. Note a key word—explain, or explanation. A theory offers a description of why something happens. A law, on the other hand, is a statement that is always true, but offers no explanation as to why. The law of gravity says a rock will fall when dropped, but does not explain why (gravitational theory is very complex and incomplete at present). The kinetic molecular theory of gases, on the other hand, states what happens when a gas is heated in a closed container (the pressure increases), but also explains why (the motions of the gas molecules are increased due to the change in temperature). Theories do not get "promoted" to laws, because laws do not answer the "why" question. Summary • The early Greek philosophers spent their time talking about nature, but did little or no actual exploration or investigation. • Inductive reasoning - to develop a general conclusion from a collection of observations. • Deductive reasoning - to make a specific statement based on a general principle. • Scientific method - a process of observation, developing a hypothesis, and testing that hypothesis. Review 1. What was the basic shortcoming of the Greek philosophers approach to studying the material world? 2. How did Aristotle improve the approach? 3. Define “inductive reasoning” and give an example. 4. Define “deductive reasoning” and give an example. 5. What is the difference between a hypothesis and a theory? 6. What is the difference between a theory and a law?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/01%3A_Introduction_to_Chemistry/1.12%3A_Scientific_Problem_Solving.txt
Can you guess what this colorful image shows? Believe it or not, it actually depicts individual atoms of cesium (reddish-orange) on a surface of gallium arsenide molecules (blue). The image was created with an extremely powerful microscope, called a scanning tunneling microscope. This is one of a few types of microscope that can make images of atoms, the basic building blocks of matter. What’s the Matter? Matter is all the “stuff” that exists in the universe. Everything you can see and touch is made of matter, including you! The only things that aren’t matter are forms of energy, such as light and sound. In science, matter is defined as anything that has mass and volume. Mass and volume measure different aspects of matter. Mass Mass is a measure of the amount of matter in a substance or an object. The basic SI unit for mass is the kilogram (kg), but smaller masses may be measured in grams (g). To measure mass, you would use a balance. In the lab, mass may be measured with a triple beam balance or an electronic balance, but the old-fashioned balance pictured below may give you a better idea of what mass is. If both sides of this balance were at the same level, it would mean that the fruit in the left pan has the same mass as the iron object in the right pan. In that case, the fruit would have a mass of 1 kg, the same as the iron. As you can see, however, the fruit is at a higher level than the iron. This means that the fruit has less mass than the iron, that is, the fruit's mass is less than 1 kg. Q: Refer to the picture above. If the fruit were at a lower level than the iron object, what would be the mass of the fruit? A: The mass of the fruit would be greater than 1 kg. Mass vs. Weight Mass is commonly confused with weight. The two are closely related, but they measure different things. Whereas mass measures the amount of matter in an object, weight measures the force of gravity acting on an object. The force of gravity on an object depends on its mass but also on the strength of gravity. If the strength of gravity is held constant (as it is all over Earth), then an object mass is directly proportional to the objects weight, so a greater mass also has a greater weight. Q: With Earth’s gravity, an object with a mass of 1 kg has a weight of 2.2 lb. How much does a 10 kg object weigh on Earth? A: A 10 kg object weighs ten times as much as a 1 kg object: 10 × 2.2 lb = 22 lb Volume Volume is a measure of the amount of space that a substance or an object takes up. The basic SI unit for volume is the cubic meter (m3), but smaller volumes may be measured in cm3, and liquids may be measured in liters (L) or milliliters (mL). How the volume of matter is measured depends on its state. • The volume of a liquid is measured with a measuring container, such as a measuring cup or graduated cylinder. • The volume of a gas depends on the volume of its container: gases expand to fill whatever space is available to them. • The volume of a regularly shaped solid can be calculated from its dimensions. For example, the volume of a rectangular solid is the product of its length, width, and height. • The volume of an irregularly shaped solid can be measured by the displacement method. You can read below how this method works. Calculating Volume from Dimensions Q: How could you find the volume of air in an otherwise empty room? A: If the room has a regular shape, you could calculate its volume from its dimensions. For example, the volume of a rectangular room can be calculated with the formula: Volume = length × width × height If the length of the room is 5.0 meters, the width is 3.0 meters, and the height is 2.5 meters, then the volume of the room is: Volume = 5.0 m × 3.0 m × 2.5 m = 37.5 m3 Measuring Volume Using the Displacement Method The following video shows how the volume of an irregular shaped object, like your science teacher can be measured by the displacement method. Q: What is the volume of the dinosaur in the diagram below? A: The volume of the water alone is 4.8 mL. The volume of the water and dinosaur together is 5.6 mL. Therefore, the volume of the dinosaur alone is 5.6 mL – 4.8 mL = 0.8 mL. Summary • Matter is all the “stuff” that exists in the universe. It has both mass and volume. • Mass measures the amount of matter in a substance or an object. The basic SI unit for mass is the kilogram (kg). • Volume measures the amount of space that a substance or an object takes up. The basic SI unit for volume is the cubic meter (m3). Review 1. How do scientists define matter? 2. What is mass? What is the basic SI unit of mass? 3. What does volume measure? Name two different units that might be used to measure volume. 4. Explain how to use the displacement method to find the volume of an irregularly shaped object.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.01%3A_Matter_Mass_and_Volume.txt
Have you ever had the experience of opening the refrigerator and seeing something that you don't recognize? Not an unfamiliar food, but something that has changed over the time period it was in the refrigerator. You are not sure just what it is, so you call it "stuff". Likely, after looking at it, you know it's not a good idea to eat it, so you throw it out. If you're not sure just what it is, you certainly do not want to put it in your body. Substances Silver and ordinary table salt are two examples of chemical substances. A substance is matter that has a uniform and definite composition. All samples of substances, sometimes called pure substances, have identical properties. When chemists run a chemical reaction, they want to use pure materials so they know exactly what they are dealing with. They know that the reaction involves a specific substance, so they expect the same reaction to give the same results each time that it is run. One characteristic of a substance is that it cannot be separated into components by physical separation techniques. The compound acetylsalicylic acid (the active ingredient in aspirin) is a substance because it is a chemical compound that cannot be further reduced to other materials by physical means. However, the aspirin that contains the acetylsalicylic acid is a mixture. In addition to the active pain-relieving ingredient, aspirin also may contain waxes, corn starch, and cellulose, among other ingredients. These added materials bind the tablet components into a solid pill and influence how rapidly the material dissolves. Substances may be either elements or pure compounds. They may also be solid, liquid, or gas. Some of the elements (such as silver) exist in the solid form. Other elements (mercury) exist as liquids at room temperature. Elements such as helium and radon exist as gases at room temperature. A compound such as water will take the form of a solid, a liquid, or a gas, depending on the temperature. Summary • A substance is a pure material with a uniform and definite composition. • A substance cannot be separated into components using physical means. • A substance can be a solid, a liquid, or a gas, dependent on temperature. Review 1. Define substance. 2. Can salt be a substance? 3. Is aspirin a substance? 2.03: Physical Properties Why are drag car standards constantly reinforced? Drag racing is a highly competitive (and expensive) sport. There are a variety of classes of vehicles, ranging from stock classes (dependent on car weight, engine size, and degree of engine modification) all the way up to the Top Fuel class. Cars in the Top Fuel class may weigh over two thousand pounds, and are capable of top speeds of well over 300 miles per hour at the end of the quarter-mile. The standards for each class are well-defined, and frequent checks are made of engine dimensions and components to ensure that the rules are followed. Physical Properties A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Silver is a shiny metal that conducts electricity very well. It can be molded into thin sheets—a property called malleability. Salt is dull, brittle, and conducts electricity when it has been dissolved into water (which it does quite easily). Physical properties of matter include color, hardness, malleability, solubility, electrical conductivity, density, melting point, and boiling point. For the elements, color does not vary much from one element to the next. The vast majority of elements are colorless, silver, or gray. Some elements do have distinctive colors: sulfur and chlorine are yellow, copper is (of course) copper-colored, and elemental bromine is red. Density can be a very useful parameter for identifying an element. Of the materials that exist as solids at room temperature, iodine has a very low density compared to zinc, chromium, and tin. Gold has a very high density, as does platinum. Hardness helps determine how an element (especially a metal) might be used. Many elements are fairly soft (silver and gold, for example) while others (such as titanium, tungsten, and chromium) are much harder. Carbon is an interesting example of hardness. In graphite (the "lead" found in pencils) the carbon is very soft, while the carbon in a diamond is roughly seven times as hard. Melting point and boiling point are somewhat unique identifiers, especially in the case of compounds. In addition to giving some idea as to the identity of the compound, important information can be obtained about the purity of the material. Is that Really Gold? Archimedes and Density How do you determine the density of an irregularly shaped object? Learn how Archimedes did this thousands of years ago with just a beaker, water, and some simple math. Summary • A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. • Physical properties include color, density, hardness, melting point, and boiling point. Review 1. What is a physical property? 2. What color are most metals? 3. Is titanium harder or softer than gold? 2.04: Extensive and Intensive Properties How much is twenty dollars really worth? You agree to mow someone's lawn for twenty dollars (it's a fairly large yard). When they pay you, they give you a \$20 bill. It doesn't matter whether the bill is brand new or old, dirty, and wrinkled—all of these bills have the same value of \$20. If you want more \$20 bills, you have to mow more lawns. You can't say, "this particular bill is actually worth more than \$20." To have more money, you have to put in more work. Extensive Properties Some properties of matter depend on the size of the sample, while some do not. An extensive property is a property that depends on the amount of matter in a sample. The mass of an object is a measure of the amount of matter that an object contains. A small sample of a certain type of matter will have a small mass, while a larger sample will have a greater mass. Another extensive property is volume. The volume of an object is a measure of the space that is occupied by that object. The figure below illustrates the extensive property of volume. The pitcher and glass both contain milk. The pitcher holds approximately two quarts and the glass will hold about 8 ounces of milk. The same milk is in each container. The only difference is the amount of milk contained in the glass and in the pitcher. Intensive Properties The electrical conductivity of a substance is a property that depends only on the type of substance. Silver, gold, and copper are excellent conductors of electricity, while glass and plastic are poor conductors. A larger or smaller piece of glass will not change this property. An intensive property is a property of matter that depends only on the type of matter in a sample and not on the amount. Other intensive properties include color, temperature, density, and solubility. The copper wire shown in the picture below has a certain electrical conductivity. You could cut off the small end that sticks out, and it would have the same conductivity as the entire long roll of wire shown here. The conductivity is a property of the copper metal itself, not of the length of the wire. Summary • An extensive property is a property that depends on the amount of matter in a sample. • Mass and volume are examples of extensive properties. • An intensive property is a property of matter that depends only on the type of matter in a sample and not on the amount. • Color, temperature, and solubility are examples of intensive properties. Review 1. Define extensive property. 2. Give two examples of extensive properties. 3. Define intensive property. 4. Give two examples of intensive properties.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.02%3A_Pure_Substances.txt
Why is the state of water different in each picture? Water can take many forms. At low temperatures (below $0^\text{o} \text{C}$), water is a solid. When at "normal" temperatures (between $0^\text{o} \text{C}$ and $100^\text{o} \text{C}$), it is a liquid. At temperatures above $100^\text{o} \text{C}$, water is a gas (steam). The state of water depends on the temperature. Each state (solid, liquid, and gas) has its own unique set of physical properties. Matter and its States Matter typically exists in one of three states: solid, liquid, or gas. There is a fourth state of matter called plasma, which rarely exists on earth, but we will omit this from our current discussion. The state a given substance exhibits is also a physical property. Some substances exist as gases at room temperature (oxygen and carbon dioxide), while others, like water and mercury metal, exist as liquids. Most metals exist as solids at room temperature. All substances can exist in any of these three states. Liquid Liquids have the following characteristics: • No definite shape (takes the shape of its container). • Has definite volume. • Particles are free to move over each other, but are still attracted to each other. A familiar liquid is mercury metal. Mercury is an anomaly. It is the only metal we know of that is liquid at room temperature. Mercury also has an ability to stick to itself (surface tension), which is a property that all liquids exhibit. Mercury has a relatively high surface tension, and this makes it very unique. Here you can see mercury in its common liquid form. If we heat liquid mercury to its boiling point of $357^\text{o} \text{C}$, and contain it under the right pressure conditions, we would notice all particles in the liquid state go into the gas state. Gas Gases have the following characteristics: • No definite shape (takes the shape of its container). • No definite volume. • Particles move in random motion with little or no attraction to each other. • Highly compressible. Solid Solids are defined by the following characteristics: • Definite shape (rigid). • Definite volume. • Particles vibrate around fixed axes. If we were to cool a sample of liquid mercury to its freezing point of $-39^\text{o} \text{C}$, and had it contained under the right pressure conditions, we would notice all of the liquid particles would go into the solid state. As you can see in the video, mercury can be solidified when its temperature is brought to its freezing point. However, when returned to room temperature conditions, mercury does not exist in solid state for long, and returns back to its more common liquid form. Plasma Plasma is a state of matter that resembles a gas but has certain properties that gases do not have. Like a gas, plasma consists of particles of matter that can pull apart and spread out, so it lacks a fixed volume and a fixed shape. Unlike a gas, plasma can conduct electricity and respond to a magnetic field. That’s because plasma consists of electrically charged particles called ions, instead of uncharged particles such as atoms or molecules. Plasma are defined by the following characteristics: • particles are charged ions and free electrons • no definite shape • no definite volume • conducts electricity • responds to magnetic field Summary • Three states of matter exist: solid, liquid, and gas. • Solids have a definite shape and volume. • Liquids have a definite volume, but take the shape of their container. • Gases have no definite shape or volume. Review 1. How many states of matter are there? 2. What is a solid? 3. What is a liquid? 4. What is a gas? 2.06: Physical Change How does a car go from new and shiny to old and beaten up? Want to buy a car – cheap? Notice there is no specification such as “in good condition” or “needs a little work.” The car above is pretty beat up. The body is damaged, the windows are broken, and the interior is probably torn up. But this is still a car. It has all the components of a car, even though you would not want to buy it in the present condition. But change that condition and you have a (possibly) useable car. Physical Change As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change. A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible. Other changes of state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state. The salt may be regained by boiling off the water, leaving the salt behind. When a piece of wood is ground into sawdust, that change is irreversible since the sawdust cannot be reconstituted into the same piece of wood that it was before. Cutting grass or pulverizing a rock are examples of irreversible physical changes. Chopping wood for a fire also represents an irreversible physical change, since the pieces cannot be put back together to form the tree. Physical Changes vs. Chemical Changes What is the difference between physical and chemical changes? Watch the video to find out. Summary • A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. • A physical change can be reversible where the original form of the matter can be restored, or irreversible where the original form cannot be restored. • Melting ice and grinding wood into sawdust are examples of physical changes. Review 1. Define physical change. 2. Why is melting an ice cube a reversible physical change? 3. Give an example of an irreversible physical change.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.05%3A_States_of_Matter.txt
Ahhhh! A tall glass of ice-cold lemonade is really refreshing on a hot day. Lemonade is a combination of lemon juice, water, and sugar. Do you know what kind of matter lemonade is? It’s obviously not an element because it consists of more than one substance. Is it a compound? Not all combined substances are compounds. Some—including lemonade—are mixtures. What Is a Mixture? mixture is a combination of two or more substances in any proportion. This is different from a compound, which consists of substances in fixed proportions. The substances in a mixture also do not combine chemically to form a new substance, as they do in a compound. Instead, they just intermingle and keep their original properties. The lemonade pictured above is a mixture because it doesn’t have fixed proportions of ingredients. It could have more or less lemon juice, for example, or more or less sugar, and it would still be lemonade. Q: What are some other examples of mixtures? A: Other examples of liquid mixtures include salt water and salad dressing. Air is a mixture of gases, mainly nitrogen and oxygen. The rock pictured in the figure below is a solid mixture. Homogeneous or Heterogeneous? The lemonade in the opening picture is an example of a homogeneous mixture. A homogeneous mixture has the same composition throughout. Another example of a homogeneous mixture is salt water. If you analyzed samples of ocean water in different places, you would find that the proportion of salt in each sample is the same: 3.5 percent. The rock in figure above is an example of a heterogeneous mixture. A heterogeneous mixture varies in its composition. The black nuggets, for example, are not distributed evenly throughout the rock. Types of Mixtures Mixtures have different properties depending on the size of their particles. Three types of mixtures based on particle size are solutions, suspensions, and colloids, all of which are described in the table below. Table $1$: Solutions, Suspensions, and Colloids Type of Mixture Description Solutions solution is a homogeneous mixture with tiny particles. The particles are too small to see and also too small to settle or be filtered out of the mixture. When the salt is thoroughly mixed into the water in this glass, it will form a solution. The salt will no longer be visible in the water, and it won’t settle to the bottom of the glass. Colloids colloid is a homogeneous mixture with medium-sized particles. The particles are large enough to see but not large enough to settle or be filtered out of the mixture. The gelatin in this dish is a colloid. It looks red because you can see the red gelatin particles in the mixture. However, the particles are too small to settle to the bottom of the dish. Suspensions suspension is a heterogeneous mixture with large particles. The particles are large enough to see and also to settle or be filtered out of the mixture. The salad dressing in this bottle is a suspension. It contains oil, vinegar, herbs, and spices. If the bottle sits undisturbed for very long, the mixture will separate into its component parts. That’s why you should shake it before you use it. Q: If you buy a can of paint at a paint store, a store employee may put the can on a shaker machine to mix up the paint in the can. What type of mixture is the paint? A: The paint is a suspension. Some of the components of the paint settle out of the mixture when it sits undisturbed for a long time. This explains why you need to shake (or stir) the paint before you use it. Q: The milk you buy in the supermarket has gone through a process called homogenization. This process breaks up the cream in the milk into smaller particles. As a result, the cream doesn’t separate out of the milk no matter how long it sits on the shelf. Which type of mixture is homogenized milk? A: Homogenized milk is a colloid. The particles in the milk are large enough to see—that’s why milk is white instead of clear like water, which is the main component of milk. However, the particles are not large enough to settle out of the mixture. Separating Mixtures The components of a mixture keep their own identity when they combine, so they retain their physical properties. Examples of physical properties include boiling point, ability to dissolve, and particle size. When components of mixtures vary in physical properties such as these, processes such as boiling, dissolving, or filtering can be used to separate them. Look at the figure below of the Great Salt Lake in Utah. The water in the lake is a solution of salt and water. Do you see the white salt deposits near the shore? How did the salt separate from the salt water? Water has a lower boiling point than salt, and it evaporates in the heat of the sun. With its higher boiling point, the salt doesn’t get hot enough to evaporate, so it is left behind. Q: Suppose you have a mixture of salt and pepper. What properties of the salt and pepper might allow you to separate them? A: Salt dissolves in water but pepper does not. If you mix salt and pepper with water, only the salt will dissolve, leaving the pepper floating in the water. You can separate the pepper from the water by pouring the mixture through a filter, such as a coffee filter. Q: After you separate the pepper from the salt water, how could you separate the salt from the water? A: You could heat the water until it boils and evaporates. The salt would be left behind. Summary • A mixture is a combination of two or more substances in any proportions. The substances in a mixture do not combine chemically, so they retain their physical properties. • A homogeneous mixture has the same composition throughout. A heterogeneous mixture varies in its composition. • Mixtures can be classified on the basis of particle size into three different types: solutions, suspensions, and colloids. • The components of a mixture retain their own physical properties. These properties can be used to separate the components by filtering, boiling, or other physical processes. Review 1. What is a mixture? 2. What is the difference between a homogeneous and a heterogeneous mixture? 3. Make a table to compare and contrast solutions, colloids, and suspensions. Include an example of each type of mixture in your table. 4. Iron filings are attracted by a magnet. This is a physical property of iron but not of most other materials, including sand. How could you use this difference in physical properties to separate a mixture of iron filings and sand?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.07%3A_Mixture.txt
How do you like your coffee? Many people enjoy a cup of coffee at some point during the day. Some may drink it black, while others may put cream (or some dairy substitute) and sugar in their coffee. High-end coffee drinks can be purchased at espresso stands (either sit-down or drive-through). Whatever the coffee drinker's preference, they want the coffee to be the same at the beginning as it is at the end of the drink. They don't want the components to separate out, but want their drink to be uniform from top to bottom. Mixtures Ordinary table salt is called sodium chloride. It is considered a substance because it has a uniform and definite composition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt easily dissolves in water, but salt water cannot be classified as a substance because its composition can vary. You may dissolve a small or large amount of salt into a given amount of water. A mixture is a physical blend of two or more components, each of which retains its own identity and properties in the mixture. Only the form of the salt is changed when it is dissolved into water. It retains its composition and properties. Homogenous Mixtures A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. Often it is easy to confuse a homogeneous mixture with a pure substance because they are both uniform. The difference is that the composition of the substance is always the same. The amount of salt in the salt water can vary from one sample to another. All solutions would be considered homogeneous because the dissolved material is present in the same amount throughout the solution. One characteristic of mixtures is that they can be separated into their components. Since each part of the mixture has not reacted with another part of the mixture, the identities of the different materials are unchanged. Summary • A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. • All solutions would be considered homogeneous. Review 1. What is a mixture? 2. What is a homogeneous mixture? 3. Are all solutions homogeneous mixtures? 4. Can homogeneous mixtures be separated into their components? 2.09: Heterogeneous Mixtures What is the best way to eat a bag of jelly beans? Many people open the bag and eat all the candy, no matter what flavor each piece is. Others pick through the collection. They may say, "I don't like the orange ones." Or maybe they only care for the lemon ones. There are different kinds of jelly beans in the mixture and people will eat what they want and get rid of the rest. Heterogeneous Mixtures A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetables and other components of the soup. A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. When oil and water are combined, they do not mix evenly, but instead form two separate layers. Each of the layers is called a phase. In the case of vegetable soup, one phase would be the liquid soup itself. This phase has vitamins, minerals, and other components dissolved in the water. This phase would be homogeneous. The carrots, peas, corn, or other vegetables represent other phases of the soup.The various vegetables are not mixed evenly in the soup, but are spread around at random. There are a large number of heterogeneous mixtures around us. Soil is composed of a variety of substances, and is often of different composition depending on the sample taken. One shovelful may come up with dirt and grass, while the next shovelful could contain an earthworm. Smog is another example of a heterogeneous mixture. This murky collection of pollutants can be a mixture of water and contaminants from burning gasoline or plastics, mixed with nitric oxide derivatives and ozone. You can see that the smog distribution in the air illustrated below is not evenly spread out, but varies from one part of the atmosphere to another. Simulation Whether the air is clean or smoggy, it's a collection of molecules. See what it's made of in this simulation. Summary • A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. • A phase is a separate layer in a heterogeneous mixture. Review Use the link below to answer the following questions: 1. Define a heterogeneous mixture. 2. Why is vegetable soup a heterogeneous mixture? 3. How many phases are in a heterogeneous mixture?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.08%3A_Homogeneous_Mixture.txt
How did gold miners search for gold? Beginning in the late 1840s, thousands of prospectors rushed to California in search of gold. One of the approaches taken to isolate the gold from the soil was called "panning". Dirt would be placed in the pan and covered with water. After thorough mixing, the pan is gently swirled to remove dissolved material while the heavier gold settles to the bottom of the pan. The gold is then separated from the mixture of soil and water. Separation of Mixtures Not everyone is out searching for gold (and not many of those searchers is going to get much gold, either). In a chemical reaction, it is important to isolate the component(s) of interest from all the other materials so they can be further characterized. Studies of biochemical systems, environmental analysis, pharmaceutical research – these and many other areas of research require reliable separation methods. Here are a number of common separation techniques: Chromatography Chromatography is the separation of a mixture by passing it in solution or suspension, or as a vapor (as in gas chromatography), through a medium in which the components move at different rates. Thin-layer chromatography is a special type of chromatography used for separating and identifying mixtures that are or can be colored, especially pigments. Distillation Distillation is an effective method to separate mixtures that are comprised of two or more pure liquids. Distillation is a purification process where the components of a liquid mixture are vaporized and then condensed and isolated. In simple distillation, a mixture is heated, and the most volatile component vaporizes at the lowest temperature. The vapor passes through a cooled tube (a condenser), where it condenses back into its liquid state. The condensate that is collected is called distillate. In the figure above, we see several important pieces of equipment. There is a heat source, a test tube with a one-hole stopper attached to a glass elbow, and rubber tubing. The rubber tubing is placed into a collection tube, which is submerged in cold water. There are other more complicated assemblies for distillation that can also be used, especially to separate mixtures which are comprised of pure liquids, with boiling points that are close to one another. Evaporation Evaporation is a technique used to separate out homogeneous mixtures that contain one or more dissolved salts. The method drives off the liquid components from the solid components. The process typically involves heating the mixture until no more liquid remains. Prior to using this method, the mixture should only contain one liquid component, unless it is not important to isolate the liquid components. This is because all liquid components will evaporate over time. The evaporation method is suitable to separate a soluble solid from a liquid. In many parts of the world, table salt is obtained from the evaporation of sea water. The heat for the process comes from the sun. Filtration Filtration is a separation method used to separate out pure substances in mixtures comprised of particles—some of which are large enough in size to be captured with a porous material. Particle size can vary considerably, given the type of mixture. For instance, stream water is a mixture that contains naturally occurring biological organisms like bacteria, viruses, and protozoa. Some water filters can filter out bacteria, the length of which are on the order of 1 micron. Other mixtures, like soil, have relatively large particle sizes, which can be filtered through something like a coffee filter. Summary • Mixtures can be separated using a variety of techniques. • Chromatography involves solvent separation on a solid medium. • Distillation takes advantage of differences in boiling points. • Evaporation removes a liquid from a solution to leave a solid material. • Filtration separates solids of different sizes. Review 1. Why is it important to separate materials from a mixture? 2. What is chromatography? 3. What is distillation 4. What is filtration? 5. What is evaporation? 6. What technique would you use to separate sand from water? There are two possibilities. 7. What technique would you use to separate alcohol from water?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.10%3A_Separating_Mixtures.txt
Who is Sherlock Holmes? The famous fictional British detective Sherlock Holmes supposedly often made the statement "elementary, my dear Watson". In reality, the closest he ever came to that line was an exchange with Watson in the short story "The Crooked Man". Holmes demonstrated shrewd insight into Watson's activities of the day. When asked how he knew what Watson was doing, Holmes simply replies "Elementary". Regardless of exactly how stated it, Sherlock Holmes was simply referring to what the Free Dictionary defines as "relating to, or constituting the basic, essential, or fundamental part". Elements An element is the simplest form of matter that has a unique set of properties. It is a pure substance that cannot be broken down into different types of substances. As of 2010, there are 118 known elements. Ninety eight of the elements are naturally occurring on earth, while the other 20 elements have been made synthetically in a lab. The names of all the elements can be found on the periodic table. Examples of well-known elements include oxygen, iron, and gold (see the figure below). Elements cannot be broken down into a simpler substance. Likewise, one element cannot be chemically converted into a different element. Some elements have been known for centuries (gold, silver, iron, and copper, for example), while others have been created in a lab only within the last several years. Most elements do not exist as such in nature. They are so reactive that they can be found only in combination with other materials. Some elements are very valuable, while others are quite inexpensive. Gold is currently worth almost \$1700 per ounce. Aluminum, on the other hand, only sells for about 90 cents per pound, considerably lower than gold. Copper is worth somewhat more, selling for approximately \$3.50 per pound. Platinum is very valuable at about \$1650 an ounce, though not quite as expensive as gold. Simulation There are elements all around you, including in the air you breathe. See what elements are in the air in this simulation. Video: The Chemical Elements Song The Chemical Elements song, sung by Tom Lehrer. At the time, there were only 102 elements discovered. Summary • An element is the simplest form of matter that has a unique set of properties. • One element cannot be chemically converted to another element. Review 1. What is an element? 2. Give the names of any two elements. 3. What is the name of the table that has the names of the elements? 4. How many elements are naturally occurring? 2.12: Compounds What do you need to start building a house? Houses take on a variety of shapes. Some are large mansions while others are simpler dwellings. No matter what the house looks like there will be certain common components – wood, brick, plaster, nails, plumbing, wiring, etc. These components go together in numerous ways to make up the unique final product. Compounds in chemistry are composed of different types and numbers of atoms. But atoms are common building blocks for a wide variety of compounds, just as brick, wood, and other materials are common building blocks for many types of houses. Compounds A compound is a substance that contains two or more elements chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest is called methane, in which there are always four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances—carbon and hydrogen. Recall that the components of a mixture can be separated from one another by physical means. This is not true for a compound. Table salt is a compound consisting of equal parts of the elements sodium and chlorine. Salt cannot be separated into its two elements by filtering, distillation, or any other physical process. Salt and other compounds can only be decomposed into their elements by a chemical process. A chemical change is a change that produces matter with a different composition. Many compounds can be decomposed into their elements by heating. When sugar is heated, it decomposes into carbon and water. Water is still a compound, but one which cannot be broken down into hydrogen and oxygen by heating. Instead, the passage of an electrical current through water will produce hydrogen and oxygen gases. The properties of compounds are generally very different than the properties of the elements from which the compound is formed. Sodium is an extremely reactive soft metal that cannot be exposed to air or water. Chlorine is a deadly gas. The compound sodium chloride is a white solid which is essential for all living things (see below). Science Friday: Celebrating Explosive Chemistry July Fourth: A day for picnics, parades and chemistry. In this video by Science Friday, Professor Bassam Shakhashiri explains some of the science of fireworks. Summary • A compound is a substance that contains two or more elements chemically combined in a fixed proportion. • A chemical change is a change that produces matter with a different composition. Review 1. What is a compound? 2. How is a compound different from an element? 3. What is a chemical change? Explore More Use the resource below to answer the following questions. 1. What lists all the elements we know about? 2. What compound is made of two hydrogen atoms and one oxygen atom? 3. What compound is made of one carbon atom and two oxygen atoms?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.11%3A_Elements.txt
Does the term chemical reaction bring to mind an image like this one? In the picture, a chemist is mixing chemicals in a lab. Many chemical reactions take place in labs. However, most chemical reactions do not. Where do they occur? They happen in the world all around you. They even happen inside your own body. In fact, you are alive only because of the many chemical reactions that constantly take place inside your cells. What Is a Chemical Reaction? chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right). Reactants → Products Chemical reactions may occur quickly or slowly. Look at the two pictures in the figure below. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust. Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from? A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form. Same Atoms, New Bonds The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products. Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The figure below represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water. Types of Chemical Reactions The chemical reaction in the figure above, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The table below compares these four types of chemical reactions. Four Types of Chemical Reactions Type of Reaction General Equation Example Synthesis A+B → C 2Na + Cl2 → 2NaCl Decomposition AB → A + B 2H2O →2H2 + O2 Single Replacement A+BC → B+ AC 2K + 2H2O → 2KOH + H2 Double Replacement AB+ CDAD + CB NaCl+ AgF → NaF + AgCl Combustion fuel + oxygen → carbon dioxide + water CH4 + 2O2 → CO2 + 2H2O Q: The burning of wood is a chemical reaction. Which type of reaction is it? A: The burning of wood—or of anything else—is a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH4). Energy In and Energy Out All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions. • In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. • In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment. Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell? A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire. Summary • A chemical reaction is a process in which some substances, called reactants, change into different substances, called products. During the reaction, chemical bonds break in the reactants and new chemical bonds form in the products. • Types of chemical reactions include synthesis, decomposition, replacement, and combustion reactions. • All chemical reactions involve energy. Exothermic reactions release more energy than they use. Endothermic reactions use more energy than they release. Review 1. What is a chemical reaction? 2. Write a general chemical equation that shows the relationship of products to reactants in a chemical reaction. 3. Contrast exothermic and endothermic chemical reactions.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.13%3A_Chemical_Reaction.txt
Do you like to cook? Cooking is a valuable skill that can benefit everyone. Whether it is fixing a simple grilled cheese sandwich or preparing an elaborate meal, cooking demonstrates some basic concepts of chemistry. When you bake bread, you mix flour, sugar, yeast, and water together. After baking, this mixture has been changed to form bread, another substance that has different characteristics and qualities from the original materials. The process of baking has produced chemical changes in the ingredients that result in bread being made. Chemical Change Most of the elements we know about do not exist freely in nature. Sodium cannot be found by itself (unless we prepare it in the laboratory) because it interacts too easily with other materials. On the other hand, the element helium does not interact with other elements to any extent. We can isolate helium from natural gas during the process of drilling for oil. A chemical change produces different materials than the ones we started with. One aspect of the science of chemistry is the study of the changes that matter undergoes. If chemistry was simply a study of elements that did nothing, life would be very boring (in fact, life would not exist since the elements are what make up our bodies and sustain us). But the processes of change that take place when different chemicals are combined produce all the materials that we use daily. One type of chemical change (already mentioned) is when two elements combine to form a compound. Another type involves the breakdown of a compound to produce the elements that make it up. If we pass an electric current through bauxite (aluminum oxide, the raw material for aluminum metal), we get metallic aluminum as a product. However, the vast majority of chemical changes involve one compound being transformed into another compound. There are literally millions of possibilities when we take this approach to chemical change. New compounds can be made to produce better fabrics that are easier to clean and maintain; they can help preserve food so it doesn’t spoil as quickly; we can make new medicines to treat diseases; even products, such as airbags (as seen in the simulation below), use a chemical change to improve the world around us. Summary A chemical change produces different materials than the ones we started with. Review 1. What is a chemical change? 2. List three types of chemical changes. 2.15: Chemical Symbols and Formulas How do chess players monitor their moves in a game? Suppose you were walking along and noticed a piece of paper on the ground with markings on it. You pick it up and see the paper in the picture above. To most people, these notes are meaningless (maybe they're a secret spy code). But to a chess player, these symbols tell the story of a chess game. Each abbreviation describes a chess piece or a move during the game. The use of special symbols allows chess players to "see" the game without having to read a wordy and possibly incomplete description of what happened. Chemical Symbols and Formulas Chemical Formula In order to illustrate chemical reactions and the elements and compounds involved in them, chemists use symbols and formulas. A chemical symbol is a one- or two-letter designation of an element. Some examples of chemical symbols are $\ce{O}$ for oxygen, $\ce{Zn}$ for zinc, and $\ce{Fe}$ for iron. The first letter of a symbol is always capitalized. If the symbol contains two letters, the second letter is lower case. The majority of elements have symbols that are based on their English names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin names, as shown below. Chemical Symbol Name Latin Name Table 1: Symbols and Latin Names for Elements $\ce{Na}$ Sodium Natrium $\ce{K}$ Potassium Kalium $\ce{Fe}$ Iron Ferrum $\ce{Cu}$ Copper Cuprum $\ce{Ag}$ Silver Argentum $\ce{Sn}$ Tin Stannum $\ce{Sb}$ Antimony Stibium $\ce{Au}$ Gold Aurum $\ce{Pb}$ Lead Plumbum Watch the following video to learn more about the origin of different element's symbols. Chemical Formulas Compounds are combinations of two or more elements. A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. Water is composed of hydrogen and oxygen in a two to one ratio. The chemical formula for water is $\ce{H_2O}$. Sulfuric acid is one of the most widely produced chemicals in the United States and is composed of the elements hydrogen, sulfur, and oxygen. The chemical formula for sulfuric acid is $\ce{H_2SO_4}$. Summary • A chemical symbol is a one- or two-letter designation of an element. • Compounds are combinations of two or more elements. • A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. • Some elements have symbols that derive from the Latin name for the element. Review 1. What is a chemical symbol? 2. What is a chemical formula? 3. How many hydrogen atoms are in one molecule of the compound C12H22O11? 4. What is the Latin name for the element potassium?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.14%3A_Chemical_Change.txt
Have you ever left your bicycle out in the rain? It's not a good idea because the rain can cause the bicycle to rust. You start to get a reddish-orange build-up of a rough deposit on the metal. It may start with the chain, but can spread to other parts of the bicycle, especially if there are scratches that create a bare metal surface. The formation of rust is a chemical process that takes place when iron is exposed to water and oxygen. It is estimated that damage due to rust costs U.S. businesses, military, and government over 276 billion dollars a year—a very expensive chemical process. Chemical Properties A chemical property describes the ability of a substance to undergo a specific chemical change. A chemical property of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust. The more general term for rusting and other similar processes is corrosion. Other terms that are commonly used to describe chemical changes are: burn, rot, explode, decompose, and ferment. Chemical properties are very useful in identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance. Chemical Change A chemical change is also called a chemical reaction. A chemical reaction is a process that occurs when one or more substances are changed into one or more new substances. Zinc $\left( \ce{Zn} \right)$ is a silver gray element that can be ground into a powder. If zinc is mixed at room temperature with powdered sulfur $\left( \ce{S} \right)$, a bright yellow element, the result will simply be a mixture of zinc and sulfur. No chemical reaction occurs. However, if energy is provided to the mixture in the form of heat, the zinc will chemically react with the sulfur to form the compound zinc sulfide $\left( \ce{ZnS} \right)$. The figure below shows the substances involved in this reaction. Alkaline metals are highly reactive with water. Watch the video to see what happens when different alkaline metals are exposed to water. Summary • A chemical property describes the ability of a substance to undergo a specific chemical change. • A chemical reaction is a process that occurs when one or more substances are changed into one or more new substances. Review 1. What is a chemical property? 2. What is a chemical reaction? 3. When can we observe a chemical property? 4. Is freezing water a chemical property? 2.17: Reactants and Products What would you do without a computer? Over the last few decades, computers have proven to be extremely useful tools for organizing and processing information. Data, such as the structural parameters of a compound, is put into the computer; the output can be a detailed diagram of how that molecule looks in three dimensions. The computer and its programs transform input data into a useful final product. Chemical Equation The reaction between zinc and sulfur can be shown in what is called a chemical equation. In words, we could write the reaction as: $\text{zinc} + \text{sulfur} \rightarrow \text{zinc sulfide}\nonumber$ The more convenient way to express a chemical reaction is to use the symbols and formulas of the substances involved: $\ce{Zn} + \ce{S} \rightarrow \ce{ZnS}\nonumber$ The substance(s) to the left of the arrow in a chemical equation are called reactants. A reactant is a substance that is present at the start of a chemical reaction. The substance(s) to the right of the arrow are called products. A product is a substance that is present at the end of a chemical reaction. In the equation above, the zinc and sulfur are the reactants that chemically combine to form the zinc sulfide product. There is a standard way of writing chemical equations. The reactants are all written on the left-hand side of the equation, with the products on the right-hand side. An arrow points from the reactants to the products to indicate the direction of the reaction: $\text{reactants} \rightarrow \text{products}\nonumber$ When appropriate, a symbol may be written above or below the arrow to indicate some special circumstance. The symbol "$\Delta$" is often used to indicate that the reaction is to be heated. The presence of the arrow also indicates that the reaction goes in one direction under the conditions indicated. There are reactions which can be easily reversed, but we will not take those up right now. There are a wide variety of reactions possible: elements may form compounds (as seen in the reaction above), compounds may form elements (water will break down in the presence of an electric current to form hydrogen gas and oxygen gas) or compounds may combine, break apart, or rearrange to form new materials. Summary • A chemical equation describes a chemical reaction. • Reactants are starting materials and are written on the left-hand side of the equation. • Products are the end result of the reaction and are written on the right-hand side of the equation. Review 1. What is a reactant? 2. What is a product? 3. What does a chemical equation do? 4. In the reaction sodium + water → sodium hydroxide + hydrogen, 1. what are the reactants? 2. what are the products? 2.18: Recognizing Chemical Reactions Have you ever cooked a pizza? Making a pizza can be as easy as buying a "take and bake" from a store and putting it in the oven, or as complicated as mixing the dough and loading it up with your favorite toppings before baking it. How do you know when it is done? The most obvious sign is that the crust turns light brown. The dough is no longer flexible, but much more rigid. Maybe the cheese has melted. You certainly want the pizza to be cooked, not half-raw. Recognizing Chemical Reactions How can a scientist tell if a chemical reaction is taking place? There are four visual clues that indicate that a chemical reaction is likely occurring: 1. A change in color occurs during the reaction. 2. A gas is produced during the reaction. 3. A solid product called a precipitate is produced in the reaction. 4. A transfer of energy occurs as a result of the reaction. Mercury (II) oxide is a red solid. When it is heated to a temperature above $500^\text{o} \text{C}$, it easily decomposes into mercury and oxygen gas. The red color of the mercury oxide reactant becomes the silver color of mercury. The color change is a sign that the reaction is occurring. When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced. The production of a gas is also an indication that a chemical reaction is occurring. When a colorless solution of lead (II) nitrate is added to a colorless solution of potassium iodide, a yellow solid called a precipitate is instantly produced. A precipitate is a solid product that forms from a reaction and settles out of a liquid solution. The formation of a precipitate is an indication of a chemical reaction. All chemical changes involve a transfer of energy. When zinc reacts with hydrochloric acid, the test tube becomes very warm as energy is released during the reaction. Some other reactions absorb energy. While energy changes are a potential sign of a chemical reaction, care must be taken to ensure that a chemical reaction is indeed taking place. Physical changes also involve a transfer of energy. The melting of a solid absorbs energy, while the condensation of a gas releases energy. The only way to be certain that a chemical reaction has taken place is to test the composition of the substances after the change has taken place, to see if they are different from the starting substances. Summary • There are four visual clues that indicate that a chemical reaction is likely occurring: • A change of color occurs during the reaction. • A gas is produced during the reaction. • A solid product called a precipitate is produced in the reaction. • A transfer of energy occurs as a result of the reaction. Review 1. What was the color change when mercury (II) oxide was heated? 2. What happened when zinc metal was mixed with hydrochloric acid? 3. What happens when lead nitrate and potassium iodide are mixed?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/02%3A_Matter_and_Change/2.16%3A_Chemical_Properties_and_Chemical_Reactions.txt
Measurement is the assignment of a number to a characteristic of an object or event, which can be compared with other objects or events. 03: Measurements How long is a yard? It depends on whom you ask, and when you asked the question. Today we have a standard definition of the yard, which you can see marked on every football field. If a football player moves the ball ten yards, they get a first down, and it doesn't matter whether they are playing in Los Angeles, Dallas, or Green Bay. At one time, however, that yard was arbitrarily defined as the distance from the tip of the king's nose to the end of his outstretched hand. Of course, the problem in that case is simple: new king, new distance (and then you have to re-mark all those football fields). SI Base Units All measurements depend on the use of units that are well known and understood. The English system of measurement units (inches, feet, ounces, etc.) are not used in science because of the difficulty in converting from one unit to another. The metric system is used because all metric units are based on multiples of 10, making conversions very simple. The metric system was originally established in France in 1795. The International System of Units is a system of measurement based on the metric system. The acronym SI is commonly used to refer to this system and stands for the French term, Le Système International d'Unités. The SI was adopted by international agreement in 1960 and is composed of seven base units, listed in Table $1$. Table $1$: SI Base Units of Measurement Quantity SI Base Unit Symbol Length meter $\text{m}$ Mass kilogram $\text{kg}$ Temperature kelvin $\text{K}$ Time second $\text{s}$ Amount of a Substance mole $\text{mol}$ Electric Current ampere $\text{A}$ Luminous Intensity candela $\text{cd}$ The first units are frequently encountered in chemistry. All other measurement quantities, such as volume, force, and energy, can be derived from these seven base units. The map below shows the adoption of the SI units in countries around the world. The United States has legally adopted the metric system for measurements, but does not use it in everyday practice. Great Britain and much of Canada use a combination of metric and imperial units. Summary • The SI system is based on multiples of ten. • There are seven basic units in the SI system. • Five of these seven SI units are commonly used in chemistry. Review 1. What does SI stand for? 2. When was this system adopted by the international community? 3. Which of the units are commonly used in chemistry? 3.02: Metric Prefixes What do Latin and Greek have to do with modern science? Isn’t it hard enough to learn English terms? For hundreds of years, the languages of the educated class were Latin and Greek. In part, because the literature of philosophy was Latin and Greek. Even the medieval Bibles were written in those two languages – the first English translation was in the late 1380s. Using Latin and Greek allowed scholars from different countries to communicate more easily with one another. Today we still see many Latin phrases in legal communications (“pro bono” meaning to do something “for the good” and not charge legal fees), scientific naming of biological species, and Latin is used for the annual student speech at Harvard University graduations. Not bad for a “dead” language. Metric Prefixes Conversions between metric system units are straightforward because the system is based on powers of ten. For example, meters, centimeters, and millimeters are all metric units of length. There are 10 millimeters in 1 centimeter and 100 centimeters in 1 meter. Metric prefixes are used to distinguish between units of different size. These prefixes all derive from either Latin or Greek terms. For example, mega comes from the Greek word $\mu \varepsilon \gamma \alpha \varsigma$, meaning "great". Table $1$ lists the most common metric prefixes and their relationship to the central unit that has no prefix. Length is used as an example to demonstrate the relative size of each prefixed unit. Table $1$: SI Prefixes Prefix Unit Abbreviation Meaning Example giga $\text{G}$ 1,000,000,000 1 gigameter $\left( \text{Gm} \right) = 10^9 \: \text{m}$ mega $\text{M}$ 1,000,000 1 megameter $\left( \text{Mm} \right) = 10^6 \: \text{m}$ kilo $\text{k}$ 1,000 1 kilometer $\left( \text{km} \right) = 1,000 \: \text{m}$ hecto $\text{h}$ 100 1 hectometer $\left( \text{hm} \right) = 100 \: \text{m}$ deka $\text{da}$ 10 1 dekameter $\left( \text{dam} \right) = 10 \: \text{m}$ 1 1 meter $\left( \text{m} \right)$ deci $\text{d}$ 1/10 1 decimeter $\left( \text{dm} \right) = 0.1 \: \text{m}$ centi $\text{c}$ 1/100 1 centimeter $\left( \text{cm} \right) = 0.01 \: \text{m}$ milli $\text{m}$ 1/1,000 1 millimeter $\left( \text{mm} \right) = 0.001 \: \text{m}$ micro $\mu$ 1/1,000,000 1 micrometer $\left( \mu \text{m} \right) = 10^{-6} \: \text{m}$ nano $\text{n}$ 1/1,000,000,000 1 nanometer $\left( \text{nm} \right) = 10^{-9} \: \text{m}$ pico $\text{p}$ 1/1,000,000,000,000 1 picometer $\left( \text{pm} \right) = 10^{-12} \: \text{m}$ There are more prefixes - some of them rarely used. Have you ever heard of a zeptometer? There are a couple of odd little practices with the use of metric abbreviations. Most abbreviations are lowercase. The lowercase "$\text{m}$" is used for meter, instead of "$\text{M}$". However, when it comes to volume, the base unit "liter" is abbreviated as "$\text{L}$" and not "$\text{l}$". So, 3.5 milliliters is written as $3.5 \: \text{mL}$. As a practical matter, whenever possible, you should express the units in a small and manageable number. If you are measuring the weight of a material that weighs $6.5 \: \text{kg}$, this is easier than saying that it weighs $6500 \: \text{g}$ or $0.65 \: \text{dag}$. All three are correct, but the $\text{kg}$ units in this case make for a small and easily managed number. However, if a specific problem needs grams instead of kilograms, go with the grams for consistency. Summary • Metric prefixes derive from Latin or Greek terms. • Metric prefixes are used to make the units manageable. Review 1. What is the prefix for “thousand”? 2. What is the prefix for 0.01? 3. How would you write 500 milliliters? 4. How many decimeters in one meter? 5. You have a mass that weighs 1.2 hectograms. How many grams does it weigh?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.01%3A_SI_Base_Units.txt
How far is the Sun from Earth? Astronomers are accustomed to very large numbers. While the moon is only $406,697 \: \text{km}$ from earth at its maximum distance, the sun is much further away $\left( 150 \: \text{million km} \right)$. Proxima Centauri, the star nearest the earth, is $39,900,000,000,000 \: \text{km}$ away—and this is just the start of long distances. On the other end of the scale, some biologists deal with very small numbers: a typical fungus could be as small as 30 micrometers $\left( 0.000030 \: \text{meters} \right)$ in length, and a virus might only be 0.03 micrometers $\left( 0.00000003 \: \text{meters} \right)$ long. Scientific Notation Scientific notation is a way to express numbers as the product of two numbers: a coefficient and the number 10 raised to a power. It is a very useful tool for working with numbers that are either very large or very small. As an example, the distance from Earth to the Sun is about 150,000,000,000 meters—a very large distance indeed. In scientific notation, the distance is written as $1.5 \times 10^{11} \: \text{m}$. The coefficient is the 1.5 and must be a number greater than or equal to 1, and less than 10. The power of 10, or exponent, is 11 because you would have to multiply 1.5 by $10^{11}$ to get the correct number. Scientific notation is sometimes referred as exponential notation. A summary of SI units is given in the table below. Table $1$: SI Prefixes Prefix Unit Abbreviation Exponential Factor Meaning Example giga $\text{G}$ $10^9$ 1,000,000,000 1 gigameter $\left( \text{Gm} \right) = 10^9 \: \text{m}$ mega $\text{M}$ $10^6$ 1,000,000 1 megameter $\left( \text{Mm} \right) = 10^6 \: \text{m}$ kilo $\text{k}$ $10^3$ 1,000 1 kilometer $\left( \text{km} \right) = 1,000 \: \text{m}$ hecto $\text{h}$ $10^2$ 100 1 hectometer $\left( \text{hm} \right) = 100 \: \text{m}$ deka $\text{da}$ $10^1$ 10 1 dekameter $\left( \text{dam} \right) = 10 \: \text{m}$ $10^0$ 1 1 meter $\left( \text{m} \right)$ deci $\text{d}$ $10^{-1}$ 1/10 1 decimeter $\left( \text{dm} \right) = 0.1 \: \text{m}$ centi $\text{c}$ $10^{-2}$ 1/100 1 centimeter $\left( \text{cm} \right) = 0.01 \: \text{m}$ milli $\text{m}$ $10^{-3}$ 1/1,000 1 millimeter $\left( \text{mm} \right) = 0.001 \: \text{m}$ micro $\mu$ $10^{-6}$ 1/1,000,000 1 micrometer $\left( \mu \text{m} \right) = 10^{-6} \: \text{m}$ nano $\text{n}$ $10^{-9}$ 1/1,000,000,000 1 nanometer $\left( \text{nm} \right) = 10^{-9} \: \text{m}$ pico $\text{p}$ $10^{-12}$ 1/1,000,000,000,000 1 picometer $\left( \text{pm} \right) = 10^{-12} \: \text{m}$ When working with small numbers, we use a negative exponent. So 0.1 meters is $1 \times 10^{-1}$ meters, 0.01 is $1 \times 10^{-2}$ and so forth. The table above gives examples of smaller units. Note the use of the leading zero (the zero to the left of the decimal point). That digit is there to help you see the decimal point more clearly. The figure 0.01 is less likely to be misunderstood than .01, where you may not see the decimal. Summary • Scientific notation allows us to express very large or very small numbers in a convenient way. • Scientific notation uses a coefficient (a number between 1 and 10) and a power of ten sufficient for the actual number. Review 1. What is scientific notation? 2. What do we use scientific notation for? 3. What is a leading zero? 4. Express 150,000,000 in scientific notation. 5. Express 0.000043 in scientific notation. 3.04: Length and Volume How were sailors able to measure the depths of seas? Back in the days before all the electronic gadgets for measuring depth and locating undersea objects existed, the "fathom" was the unit of measurement for depth. A rope was knotted every six feet and the end was dropped over the side of the ship. You could tell how deep the water was by how many knots went under the water before the rope hit bottom. Today we just turn on an instrument and read the depth to a high level of accuracy. Length and Volume Length is the measurement of the extent of something along its greatest dimension. The SI basic unit of length, or linear measure, is the meter $\left( \text{m} \right)$. All measurements of length may be made in meters, though the prefixes listed in various tables will often be more convenient. The width of a room may be expressed as about 5 meters $\left( \text{m} \right)$, whereas a large distance, such as the distance between New York City and Chicago, is better expressed as 1150 kilometers $\left( \text{km} \right)$. Very small distances can be expressed in units such as the millimeter or the micrometer. The width of a typical human hair is about 20 micrometers $\left( \mu \text{m} \right)$. Volume is the amount of space occupied by a sample of matter. The volume of a regular object can be calculated by multiplying its length by its width and height. Since each of those is a linear measurement, we say that units of volume are derived from units of length. The SI unit of volume is the cubic meter $\left( \text{m}^3 \right)$, which is the volume occupied by a cube that measures $1 \: \text{m}$ on each side. This very large volume is not convenient for typical use in a chemistry laboratory. A liter $\left( \text{L} \right)$ is the volume of a cube that measures $10 \: \text{cm}$ $\left( 1 \: \text{dm} \right)$ on each side. A liter is thus equal to both $1000 \: \text{cm}^3$ $\left( 10 \: \text{cm} \times 10 \: \text{cm} \times 10 \: \text{cm} \right)$, and to $1 \: \text{dm}^3$. A smaller unit of volume that is commonly used is the milliliter ($\text{mL}$—note the capital $\text{L}$, which is a standard practice). A milliliter is the volume of a cube that measures $1 \: \text{cm}$ on each side. Therefore, a milliliter is equal to a cubic centimeter $\left( \text{cm}^3 \right)$. There are $1000 \: \text{mL}$ in $1 \: \text{L}$, which is the same as saying that there are $1000 \: \text{cm}^3$ in $1 \: \text{dm}^3$. Summary • Length is the measurement of the extent of something along its greatest dimension. • Volume is the amount of space occupied by a sample of matter. • Volume can be determined by knowing the length of each side of an item. Review 1. Define length. 2. Define volume. 3. An object measures 6.2 cm×13.7 cm×26.9 cm6.2 cm×13.7 cm×26.9 cm. Which value is the length of the object? 4. How big is a mL?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.03%3A_Scientific_Notation_in_Chemistry.txt
How is he floating? One of the many interesting things about travel in outer space is the idea of weightlessness. If something is not fastened down, it will float in mid-air. Early astronauts learned that weightlessness had negative effects on bone structure. If there was no pressure on the legs, those bones would begin to lose mass. Weight provided by gravity is needed to maintain healthy bones. Specially designed equipment is now a part of every space mission, so the astronauts can maintain good body fitness. Mass and Weight Mass is a measure of the amount of matter that an object contains. The base SI unit of mass is the kilogram or kg, which was originally defined as the mass of $1 \: \text{L}$ of liquid water at $4^\text{o} \text{C}$ (the volume of a liquid changes slightly with temperature). The kilogram is the only SI base unit that contains a prefix. This is because when the first physical standard for mass was created in 1799, the methods to measure the mass of something as small as a gram did not exist. In the laboratory, mass is measured with a balance (figure below), which must be calibrated with a standard mass so that its measurements are accurate. Other common units of mass are the gram and the milligram. A gram is 1/1000th of a kilogram, meaning that there are $1000 \: \text{g}$ in $1 \: \text{kg}$. A milligram is 1/1000th of a gram, so there are $1000 \: \text{mg}$ in $1 \: \text{g}$. Mass is often confused with the term weight. Weight is a measure of force that is equal to the gravitational pull on an object. The weight of an object is dependent on its location. On the moon, the force due to gravity is about one sixth that of the gravitational force on Earth. Therefore, a given object will weigh six times more on Earth than it does on the moon. Since mass is dependent only on the amount of matter present in an object, mass does not change with location. Weight measurements are often made with a spring scale by reading the distance that a certain object pulls down and stretches a spring. Summary • Mass is a measure of the amount of matter that an object contains. • Weight is a measure of force that is equal to the gravitational pull on an object. • Mass is independent of location, while weight depends on location. Review 1. Define mass. 2. Define weight. 3. If I weigh 180 pounds on Earth, what will I weigh on the moon? 3.06: Kinetic Energy Have you ever watched a cat in action? When a cat is chasing something, its moves very fast. A person may comment, "that cat has a lot of energy"—which is more correct than they might realize! One form of energy is seen when an object is moving, and this type of energy is the basis for many chemical processes. SI Kinetic Energy Units An object's kinetic energy is the energy due to motion. Kinetic energy can be defined mathematically as: $KE = \frac{1}{2} mv^2\nonumber$ where $KE$ is kinetic energy, $m$ is mass, and $v$ is velocity. Energy is defined as the capacity to do work or to produce heat. As discussed previously, kinetic energy is one type of energy and is associated with motion. Another frequently encountered energy is potential energy, a type of energy that is stored in matter and released during a chemical reaction. The joule $\left( \text{J} \right)$ is the SI unit of energy and is named after English physicist James Prescott Joule (1818-1889). If we go back to the equation for kinetic energy written above, we can put units in: $\text{kg}$ for mass and $\text{m}^2/\text{s}^2$ for velocity squared. Then, in terms of SI base units, a joule is equal to a kilogram times meter squared, divided by a second squared $\left( \text{kg} \cdot \text{m}^2/\text{s}^2 \right)$. Another common unit of energy that is often used is the calorie $\left( \text{cal} \right)$, which is equivalent to $4.184 \: \text{J}$. Summary • Energy is the capacity to do work or to produce heat. • Kinetic energy is energy due to motion. • Potential energy is energy stored in matter. • The joule $\left( \text{J} \right)$ is the SI unit of energy and equals $\text{kg} \cdot \text{m}^2/\text{s}^2$. Review 1. What is kinetic energy? 2. What is the mathematical equation for kinetic energy? 3. What is potential energy? 4. What is the SI unit for energy?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.05%3A_Mass_and_Weight.txt
An ice cube in your hand feels cold, but why? The particles of matter in a hot object are moving much faster than the particles of matter in a cold object. An object’s kinetic energy is the energy due to motion. The particles of matter that make up the hot stove have a greater amount of kinetic energy than those in the ice cube. Temperature and Temperature Scales Temperature is a measure of the average kinetic energy of the particles in matter. In everyday usage, temperature indicates a measure of how hot or cold an object is. Temperature is an important parameter in chemistry. When a substance changes from solid to liquid, it is because there was an increase in the temperature of the material. Chemical reactions usually proceed faster if the temperature is increased. Many unstable materials (such as enzymes) will be viable longer at lower temperatures. Temperature Scales The first thermometers were glass and contained alcohol, which expanded and contracted as the temperature changed. The German scientist Daniel Gabriel Fahrenheit used mercury in the tube, an idea put forth by Ismael Boulliau. The Fahrenheit scale was first developed in 1724 and tinkered with for some time after that. The main problems with this scale are the arbitrary definitions of temperature. The freezing point of water was defined as $32^\text{o} \text{F}$ and the boiling point as $212^\text{o} \text{F}$. The Fahrenheit scale is typically not used for scientific purposes. The Celsius scale of the metric system is named after Swedish astronomer Anders Celsius (1701-1744). The Celsius scale sets the freezing point and boiling point of water at $0^\text{o} \text{C}$ and $100^\text{o} \text{C}$, respectively. The distance between those two points is divided into 100 equal intervals, each of which is one degree. Another term sometimes used for the Celsius scale is "centigrade" because there are 100 degrees between the freezing and boiling points of water on this scale. However, the preferred term is "Celsius". The Kelvin temperature scale is named after Scottish physicist and mathematician Lord Kelvin (1824-1907). It is based on molecular motion, with the temperature of $0 \: \text{K}$, also known as absolute zero, being the point where all molecular motion ceases. The freezing point of water on the Kelvin scale is $273.15 \: \text{K}$, while the boiling point is $373.15 \: \text{K}$. Notice that there is no "degree" used in the temperature designation. Unlike the Fahrenheit and Celsius scales, where temperatures are referred to as "degrees $\text{F}$" or "degrees $\text{C}$", we simply designated temperatures in the Kelvin scale as kelvins. As can be seen by the 100 kelvin difference between the two, a change of one degree on the Celsius scale is equivalent to the change of one kelvin on the Kelvin scale. Converting from one scale to another is easy, as you simply add or subtract 273. Simulation No matter the temperature scale you use, the temperature at which a liquid boils depends on a few different variables. Try boiling water in a few places around the world to see what we mean. Summary • Temperature is a measure of the average kinetic energy of the particles in matter. • The Fahrenheit scale defines the freezing point of water as $32^\text{o} \text{F}$ and the boiling point as $212^\text{o} \text{F}$. • The Celsius scale sets the freezing point and boiling point of water at $0^\text{o} \text{C}$ and $100^\text{o} \text{C}$, respectively. • The Kelvin scale is based on molecular motion, with the temperature of $0 \: \text{K}$—also known as absolute zero—being the point where all molecular motion ceases. Review 1. What is absolute zero on the Celsius temperature scale? 2. What are the freezing and boiling points of water in the Celsius scale? 3. Convert the following Kelvin temperatures to degrees Celsius. 1. 188 K 2. 631 K 4. Temperature in degrees Fahrenheit can be converted to Celsius by first subtracting 32, then dividing by 1.8. What is the Celsius temperature outside on a warm day (88°F)? 5. Why is the Celsius scale sometimes called “centigrade”?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.07%3A_Temperature_and_Temperature_Scales.txt
Conversion Factors Many quantities can be expressed in several different ways. The English system measurement of 4 cups is also equal to 2 pints, 1 quart, and $\frac{1}{4}$ of a gallon. $4 \: \text{cups} = 2 \: \text{pints} = 1 \: \text{quart} = 0.25 \: \text{gallon}\nonumber$ Notice that the numerical component of each quantity is different, while the actual amount of material that it represents is the same. This is because the units are different. We can establish the same set of equalities for the metric system: $1 \: \text{meter} = 10 \:\text{decimeters} = 100 \: \text{centimeters} = 1000 \: \text{millimeters}\nonumber$ The metric system's use of powers of 10 for all conversions makes this quite simple. Whenever two quantities are equal, a ratio can be written that is numerically equal to 1. Using the metric examples above: $\frac{1 \: \text{m}}{100 \: \text{cm}} = \frac{100 \: \text{cm}}{100 \: \text{cm}} = \frac{1 \: \text{m}}{1 \: \text{m}} = 1\nonumber$ The $\frac{1 \: \text{m}}{100 \: \text{cm}}$ is called a conversion factor. A conversion factor is a ratio of equivalent measurements. Because both $1 \: \text{m}$ and $100 \: \text{cm}$ represent the exact same length, the value of the conversion factor is 1. The conversion factor is read as "1 meter per 100 centimeters". Other conversion factors from the cup measurement example can be: $\frac{4 \: \text{cups}}{2 \: \text{pints}} = \frac{2 \: \text{pints}}{1 \: \text{quart}} = \frac{1 \: \text{quart}}{0.25 \: \text{gallon}} = 1\nonumber$ Since the numerator and denominator represent equal quantities in each case, all are valid conversion factors. Scientific Dimensional Analysis Conversion factors are used in solving problems in which a certain measurement must be expressed with different units. When a given measurement is multiplied by an appropriate conversion factor, the numerical value changes, but the actual size of the quantity measured remains the same. Dimensional analysis is a technique that uses the units (dimensions) of the measurement in order to correctly solve problems. Dimensional analysis is best illustrated with an example. Example $1$ How many seconds are in a day? Known • 1 day = 24 hours • 1 hour = 60 minutes • 1 minute = 60 seconds Unknown The known quantities above represent the conversion factors that we will use. The first conversion factor will have day in the denominator so that the "day" unit will cancel. The second conversion factor will then have hours in the denominator, while the third conversion factor will have minutes in the denominator. As a result, the unit of the last numerator will be seconds and that will be the units for the answer. Step 2: Calculate. $1 \: \text{d} \times \frac{24 \: \text{hr}}{1 \: \text{d}}\times \frac{60 \: \text{min}}{1 \: \text{hr}} \times \frac{60 \: \text{s}}{1 \: \text{min}} = 86,400 \: \text{s} \nonumber\nonumber$ Applying the first conversion factor, the "$\text{d}$" unit cancels and $1 \times 24 = 24$. Applying the second conversion factor, the "$\text{hr}$" unit cancels and $24 \times 60 = 1440$. Applying the third conversion factor, the "$\text{min}$" unit cancels and $1440 \times 60 = 86,400$. The unit that remains is "$\text{s}$" for seconds. Step 3: Think about your result. Seconds is a much smaller unit of time than days, so it makes sense that there are a very large number of seconds in one day. Summary • A conversion factor is a ratio of equivalent measurements. • Dimensional analysis is a technique that uses the units (dimensions) of the measurement to solve problems. Review 1. What is a conversion factor? 2. What is dimensional analysis? 3. How many meters are in 3.7 km? 4. How many kg in 12980 g? Explore More Use the link below to answer the following questions: http://www.felderbooks.com/papers/units.html 1. What do we always need to express measurements correctly? 2. What does dimensional analysis tell you? 3. How do you know that you have set the problem up incorrectly? 4. How do you know that you have set the problem up correctly?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.08%3A_Dimensional_Analysis.txt
How can a number of track laps be converted to a distance in meters? You are training for a 10-kilometer run by doing laps on a 400-meter track. You ask yourself how many times you need to run around the track in order to cover ten kilometers (...more than you realize). By using dimensional analysis, you can easily determine the number of laps needed to cover the $10 \: \text{km}$ distance. Metric Unit Conversions The metric system's many prefixes allow quantities to be expressed in many different units. Dimensional analysis is useful to convert from one metric system unit to another. Example $1$ A particular experiment for science class requires $120 \: \text{mL}$ of a solution. The science teacher knows that he will need to make enough solution for 40 experiments to be performed throughout the day. How many liters of solution should he prepare? Known • 1 experiment requires $120 \: \text{mL}$ • $1 \: \text{L} = 1000 \: \text{mL}$ Unknown Since each experiment requires $120 \: \text{mL}$ of solution and the teacher needs to prepare enough for 40 experiments, multiply 120 by 40 to get $4800 \: \text{mL}$ of solution needed. Now you must convert $\text{mL}$ to $\text{L}$ by using a conversion factor. Step 2: Calculate. $4800 \: \text{mL} \times \frac{1 \: \text{L}}{1000 \: \text{mL}} = 4.8 \: \text{L}\nonumber$ Note that the conversion factor is arranged so that the $\text{mL}$ unit is in the denominator and thus cancels out, leaving $\text{L}$ as the remaining unit in the answer. Step 3:Think about your result. A liter is much larger than a milliliter, so it makes sense that the number of liters required is less than the number of milliliters. Two-Step Metric Unit Conversions Some metric conversion problems are most easily solved by breaking them down into more than one step. When both the given unit and the desired unit have prefixes, one can first convert to the simple (un-prefixed) unit, followed by a conversion to the desired unit. The following example will illustrate this method. Example $2$ Convert $4.3 \: \text{cm}$ to $\mu \text{m}$. Known • $1 \: \text{m} = 100 \: \text{cm}$ • $1 \: \text{m} = 10^6 \: \mu \text{m}$ Unknown You may need to consult a table for the multiplication factor represented by each metric prefix. First convert $\text{cm}$ to $\text{m}$, then convert $\text{m}$ to $\mu \text{m}$. Step 2: Calculate. $4.3 \: \text{cm} \times \frac{1 \: \text{m}}{100 \: \text{cm}} \times \frac{10^6 \: \mu \text{m}}{1 \: \text{m}} = 43,000 \: \mu \text{m}\nonumber$ Each conversion factor is written so that the unit of the denominator cancels with the unit of the numerator of the previous factor. Step 3: Think about your result. A micrometer is a smaller unit of length than a centimeter, so the answer in micrometers is larger than the number of centimeters given. Summary • Dimensional analysis can be used to carry out metric unit conversions. Review 1. Perform the following conversions. 1. 0.074 km to m 2. 24,600 μg to g 3. 4.9 × 107 μg to kg 4. 84 dm to mm 3.10: Derived Units How has farming evolved? As farming becomes more expensive and less profitable (at least for small farms), farmers might decide to sell their land to builders who want to erect commercial or residential properties. In order to sell, an accurate property title is needed. The dimensions of the farm must be determined, and the acreage calculated from those dimensions. Dimensional Analysis and Derived Units Some units are combinations of SI base units. A derived unit is a unit that results from a mathematical combination of SI base units. We have already discussed volume and energy as two examples of derived units. Some others are listed in the table below: Derived SI Units Quantity Symbol Unit Unit Abbreviation Derivation Area $A$ square meter $\text{m}^2$ $\text{length} \times \text{width}$ Volume $V$ cubic meter $\text{m}^3$ $\text{length} \times \text{width} \times \text{height}$ Density $D$ kilograms/cubic meter $\text{kg/m}^3$ $\frac{\text{mass}}{\text{volume}}\0 Concentration \(c$ moles/liter $\text{mol/L}$ $\frac{\text{amount}}{\text{volume}}$ Speed (velocity) $v$ meters/second $\text{m/s}$ $\frac{\text{length}}{\text{time}}$ Acceleration $a$ meters/second/second $\text{m/s}^2$ $\frac{\text{speed}}{\text{time}}$ Force $F$ newton $\text{N}$ $\text{mass} \times \text{acceleration}$ Energy $E$ joule $\text{J}$ $\text{force} \times \text{length}$ Using dimensional analysis with derived units requires special care. When units are squared or cubed as with area or volume, the conversion factors themselves must also be squared. Shown below is the conversion factor for cubic centimeters and cubic meters. $\left( \frac{1 \: \text{m}}{100 \: \text{cm}} \right)^3 = \frac{1 \: \text{m}^3}{10^6 \: \text{cm}^3} = 1\nonumber$ Because a cube has 3 sides, each side is subject to the conversion of $1 \: \text{m}$ to $100 \: \text{cm}$. Since 100 cubed is equal to 1 million $\left( 10^6 \right)$, there are $10^6 \: \text{cm}^3$ in $1 \: \text{m}^3$. Two convenient volume units are the liter (which is equal to a cubic decimeter) and the milliliter, which is equal to a cubic centimeter. The conversion factor would be: $\left( \frac{1 \: \text{dm}}{10 \: \text{cm}} \right)^3 = \frac{1 \: \text{dm}^3}{1000 \: \text{cm}^3} = 1\nonumber$ There are thus $1000 \: \text{cm}^3$ in $1 \: \text{dm}^3$, which is the same thing as saying there are $1000 \: \text{mL}$ in $1 \: \text{L}$. Example $1$ Convert $3.6 \times 10^8 \: \text{mm}^3$ to $\text{mL}$. Known • $1 \: \text{m} = 1000 \: \text{mm}$ • $1 \: \text{mL} = 1 \: \text{cm}^3$ • $1 \: \text{m} = 100 \: \text{cm}$ Unknown This problem requires multiple steps and the technique for converting with derived units. Simply proceed one step at a time: $\text{mm}^3$ to $\text{m}^3$ to $\text{cm}^3 = \text{mL}$. Step 2: Calculate. $3.6 \: \text{mm}^3 \times \left( \frac{1 \: \text{m}}{1000 \: \text{mm}} \right)^3 \times \left( \frac{100 \: \text{cm}}{1 \: \text{m}} \right)^3 \times \frac{1 \: \text{mL}}{1 \: \text{cm}^3} = 0.0036 \: \text{mL} \nonumber\nonumber$ Numerically, the steps are to divide 3.6 by $10^9$, followed by multiplying by $10^6$. You may find that you can shorten the problem by a step by first determining the conversion factor from $\text{mm}$ to $\text{cm}$, and using that instead of first converting to $\text{m}$. There are $10 \: \text{mm}$ in $1 \: \text{cm}$. $3.6 \: \text{mm}^3 \times \left( \frac{1 \: \text{cm}}{10 \: \text{mm}} \right)^3 \times \frac{1 \: \text{mL}}{1 \: \text{cm}^3} = 0.0036 \: \text{mL} \nonumber\nonumber$ In this case, 3.6/1000 gives the same result of 0.0036. Step 3: Think about your result. Cubic millimeters are much smaller than cubic centimeters, so the final answer is much less than the original number of $\text{mm}^3$. Summary • A derived unit is a unit that results from a mathematical combination of SI base units. • Calculations involving derived units follow the same principles as other unit conversion calculations. Review 1. What is a derived unit? 2. Convert 0.00722 km3 to m3 3. Convert 129 cm3 to L 4. Convert 4.9 × 105 μm3 to mm3	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.09%3A_Metric_Unit_Conversions.txt
How do logs stay afloat in water? After trees are cut, logging companies often move the raw material down a river to a sawmill where it can be shaped into building materials or other products. The logs float on the water because they are less dense than the water they are in. Knowledge of density is important in the characterization and separation of materials. Information about density allows us to make predictions about the behavior of matter. Density A golf ball and a table tennis ball are about the same size. However, the golf ball is much heavier than the table tennis ball. Now imagine a similar size ball made out of lead. That would be very heavy indeed! What are we comparing? By comparing the mass of an object relative to its size, we are studying a property called density. Density is the ratio of the mass of an object to its volume. $\text{Density} = \frac{\text{mass}}{\text{volume}}\nonumber$ Density is an intensive property, meaning that it does not depend on the amount of material present in the sample. Water has a density of $1.0 \: \text{g/mL}$. That density is the same whether you have a small glass of water or a swimming pool full of water. Density is a property that is constant for the particular identity of the matter being studied. The SI unit of density is kilograms per cubic meter $\left( \text{kg/m}^3 \right)$, since the $\text{kg}$ and the $\text{m}$ are the SI units for mass and length respectively. In everyday usage in a laboratory, this unit is awkwardly large. Most solids and liquids have densities that are conveniently expressed in grams per cubic centimeter $\left( \text{g/cm}^3 \right)$. Since a cubic centimeter is equal to a milliliter, density units can also be expressed as $\text{g/mL}$. Gases are much less dense than solids and liquids, so their densities are often reported in $\text{g/L}$. Densities of some common substances at $20^\text{o} \text{C}$ are listed in the table below. Table $1$: Densities of Some Common Substances Liquids and Solids Density at $20^\text{o} \text{C} \: \left( \text{g/mL} \right)$ Gases Density at $20^\text{o} \text{C} \: \left( \text{g/L} \right)$ Ethanol 0.79 Hydrogen 0.084 Ice $\left( 0^\text{o} \text{C} \right)$ 0.917 Helium 0.166 Corn oil 0.922 Air 1.20 Water 0.998 Oxygen 1.33 Water $\left( 4^\text{o} \text{C} \right)$ 1.000 Carbon dioxide 1.83 Corn syrup 1.36 Radon 9.23 Aluminum 2.70 Copper 8.92 Lead 11.35 Mercury 13.6 Gold 19.3 Since most materials expand as temperature increases, the density of a substance is temperature dependent, and usually decreases as temperature increases. Ice floats in water, and it can be seen from the table that ice is less dense. Corn syrup, being more dense, would sink if placed in water. Example $1$ An $18.2 \: \text{g}$ sample of zinc metal has a volume of $2.55 \: \text{cm}^3$. Calculate the density of zinc. Known • Mass $= 18.2 \: \text{g}$ • Volume $= 2.55 \: \text{cm}^3$ Unknown Use the equation for density, $D = \frac{m}{V}$, to solve the problem. Step 2: Calculate. $D = \frac{m}{V} = \frac{18.2 \: \text{g}}{2.55 \: \text{cm}^3} = 7.14 \: \text{g/cm}^3\nonumber$ Step 3: Think about your result. If $1 \: \text{cm}^3$ of zinc has a mass of about 7 grams, then 2 and a half $\text{cm}^3$ will have a mass about 2 and a half times as great. Metals are expected to have a density greater than that of water, and zinc's density falls within the range of the other metals listed above. Since density values are known for many substances, density can be used to determine an unknown mass or an unknown volume. Dimensional analysis will be used to ensure that units cancel appropriately. Example $2$ 1. What is the mass of $2.49 \: \text{cm}^3$ of aluminum? 2. What is the volume of $50.0 \: \text{g}$ of aluminum? Known • Density $= 2.70 \: \text{g/cm}^3$ • 1. Volume $= 2.49 \: \text{cm}^3$ • 2. Mass $= 50.0 \: \text{g}$ Unknown • 1. Mass $= ? \: \text{g}$ • 2. Volume $= ? \: \text{cm}^3$ Use the equation for density, $D = \frac{m}{V}$, and dimensional analysis to solve each problem. Step 2: Calculate. $1. \: \: 2.49 \: \text{cm}^3 \times \frac{2.70 \: \text{g}}{1 \: \text{cm}^3} = 6.72 \: \text{g}\nonumber$ $2. \: \: 50.0 \: \text{g} \times \frac{1 \: \text{cm}^3}{2.70 \: \text{g}} = 18.5 \: \text{cm}^3\nonumber$ In problem 1, the mass is equal to the density multiplied by the volume. In problem 2, the volume is equal to the mass divided by the density. Step 3: Think about your results. Because a mass of $1 \: \text{cm}^3$ of aluminum is $2.70 \: \text{g}$, the mass of about $2.5 \: \text{cm}^3$ should be about 2.5 times larger. The $50 \: \text{g}$ of aluminum is substantially more than its density, so that amount should occupy a relatively large volume. Summary • Density is the ratio of the mass of an object to its volume. • Gases are less dense than either solids or liquids. • Both liquid and solid materials can have a variety of densities. • For liquids and gases, the temperature will affect the density to some extent. Review 1. Define “density.” 2. Are gases more or less dense that liquids or solids at room temperature? 3. How does temperature affect the density of a material? 4. A certain liquid sample has a volume of 14.7 mL and a mass of 22.8 grams. Calculate the density. 5. A material with a density of 2.7 grams/mL occupies 35.6 mL. How many grams of the material are there? 6. A certain material has a density of 19.3 g/mL. What is the material?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.11%3A_Density.txt
How do professional basketball players improve their shooting accuracy? Basketball is one of those sports where you need to hit the target. A football field goal kicker might have room for some deviation from a straight line – for college and pro football there is an 18 foot 6 inch space for the ball to go through. In basketball, the basket is only 18 inches across and the ball is a little less than 10 inches across – not much room for error. The ball has to be on target in order to go into the basket and score. Accuracy and Precision In everyday speech, the terms accuracy and precision are frequently used interchangeably. However, their scientific meanings are quite different. Accuracy is a measure of how close a measurement is to the correct or accepted value of the quantity being measured. Precision is a measure of how close a series of measurements are to one another. Precise measurements are highly reproducible, even if the measurements are not near the correct value. Darts thrown at a dartboard are helpful in illustrating accuracy and precision. Assume that three darts are thrown at the dartboard, with the bulls-eye representing the true, or accepted, value of what is being measured. A dart that hits the bulls-eye is highly accurate, whereas a dart that lands far away from the bulls-eye displays poor accuracy. The figure above demonstrates four possible outcomes: 1. The darts have landed far from each other and far from the bulls-eye. This grouping demonstrates measurements that are neither accurate nor precise. 2. The darts are close to one another, but far from the bulls-eye. This grouping demonstrates measurements that are precise, but not accurate. In a laboratory situation, high precision with low accuracy often results from a systematic error. Either the measurer makes the same mistake repeatedly, or the measuring tool is somehow flawed. A poorly calibrated balance may give the same mass reading every time, but it will be far from the true mass of the object. 3. The darts are not grouped very near to each other, but are generally centered around the bulls-eye. This demonstrates poor precision, but fairly high accuracy. This situation is not desirable in a lab situation because the "high" accuracy may simply be random chance and not a true indicator of good measuring skill. 4. The darts are grouped together and have hit the bulls-eye. This demonstrates high precision and high accuracy. Scientists always strive to maximize both in their measurements. Simulation How is your accuracy and precision? Try out your skills with this simulation. Summary • Accuracy is a measure of how close a measurement is to the correct or accepted value of the quantity being measured. • Precision is a measure of how close a series of measurements are to one another. Review 1. Define accuracy. 2. Define precision. 3. What can be said about the reproducibility of precise values? 3.13: Percent Error How does an electrical circuit work? A complicated piece of electronic equipment may contain several resistors that have the role of controlling the voltage and current in the electrical circuit. If there is too much current, the apparatus malfunctions. Conversely, too little current means the system simply will not perform. The resistor values are always given with an error range. A resistor may have a stated value of 200 ohms, but a $10\%$ error range, meaning the resistance could be anywhere between 195 and 205 ohms. By knowing these values, an electronics person can design and service the equipment to make sure that it functions properly. Percent Error An individual measurement may be accurate or inaccurate, depending on how close it is to the true value. Suppose that you are doing an experiment to determine the density of a sample of aluminum metal. The accepted value of a measurement is the true or correct value based on general agreement with a reliable reference. For aluminum, the accepted density is $2.70 \: \text{g/cm}^3$. The experimental value of a measurement is the value that is measured during the experiment. Suppose that in your experiment, you determine an experimental value for the aluminum density to be $2.42 \: \text{g/cm}^3$. The error of an experiment is the difference between the experimental and accepted values. $\text{Error} = \text{experimental value} - \text{accepted value}\nonumber$ If the experimental value is less than the accepted value, the error is negative. If the experimental value is larger than the accepted value, the error is positive. Often, error is reported as the absolute value of the difference in order to avoid the confusion of a negative error. The percent error is the absolute value of the error, divided by the accepted value, and multiplied by $100\%$. $\% \: \text{Error} = \frac{\left\| \text{experimental value} - \text{accepted value} \right\|}{\text{accepted value}} \times 100\%\nonumber$ To calculate the percent error for the aluminum density measurement, we can substitute the given values of $2.45 \: \text{g/cm}^3$ for the experimental value and $2.70 \: \text{g/cm}^3$ for the accepted value. $\% \: \text{Error} = \frac{\left\| 2.45 \: \text{g/cm}^3 - 2.70 \: \text{g/cm}^3 \right\|}{2.70 \: \text{g/cm}^3} \times 100\% = 9.26\%\nonumber$ If the experimental value is equal to the accepted value, the percent error is equal to 0. As the accuracy of a measurement decreases, the percent error of that measurement rises. Summary • Definitions of accepted value and experimental values are given. • Calculations of error and percent error are demonstrated. Review 1. Define accepted value. 2. Define experimental value 3. What happens as the accuracy of the measurement decreases?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.12%3A_Accuracy_and_Precision.txt
How do police officers identify criminals? After a bank robbery has been committed, police will ask witnesses to describe the thieves. They will usually get an answer such as "medium height". Others may say something along the lines of "between 5 foot 8 inches and 5 foot 10 inches". In both cases, there is a significant amount of uncertainty about the height of the criminals. Measurement Uncertainty Some error or uncertainty always exists in any measurement. The amount of uncertainty depends both upon the skill of the measurer and upon the quality of the measuring tool. While some balances are capable of measuring masses only to the nearest $0.1 \: \text{g}$, other highly sensitive balances are capable of measuring to the nearest $0.001 \: \text{g}$, or even better. Many measuring tools, such as rulers and graduated cylinders, have small lines which need to be carefully read in order to make a measurement. The figure below shows two rulers making the same measurement of an object (indicated by the blue arrow). With either ruler, it is clear that the length of the object is between $2$ and $3 \: \text{cm}$. The bottom ruler contains no millimeter markings. With that ruler, the tenths digit can be estimated and the length may be reported as $2.5 \: \text{cm}$. However, another person may judge that the measurement is $2.4 \: \text{cm}$ or perhaps $2.6 \: \text{cm}$. While the 2 is known for certain, the value of the tenths digit is uncertain. The top ruler contains marks for tenths of a centimeter (millimeters). With this ruler, the same object may be measured as $2.55 \: \text{cm}$. The measurer is capable of estimating the hundredths digit because he can be certain that the tenths digit is a 5. Again, another measurer may report the length to be $2.54 \: \text{cm}$ or $2.56 \: \text{cm}$. In this case, there are two certain digits (the 2 and the 5), with the hundredths digit being uncertain. Clearly, the top ruler is a superior ruler for measuring lengths as precisely as possible. Summary • Uncertainty exists in all measurements. • The degree of uncertainty is affected in part by the quality of the measuring tool. Review 1. What is uncertainty in measurements? 2. Why is the top ruler more reliable in measuring length than the bottom ruler? 3. How could the top ruler be made more accurate? 3.15: Rounding Have you ever been fishing People who fish often are a little unreliable when it comes to describing what fish they caught and how much it weighed. It's easier to say that the fish weighed ten pounds than it is to accurately describe the weight of 8 pounds 11 ounces. Ten pounds is "close enough" when a fisherman is talking about their catch. Rounding Before dealing with the specifics of the rules for determining the significant figures in a calculated result, we need to be able to round numbers correctly. To round a number, first decide how many significant figures the number should have. Once you know that, round to that many digits, starting from the left. If the number immediately to the right of the last significant digit is less than 5, it is dropped and the value of the last significant digit remains the same. If the number immediately to the right of the last significant digit is greater than or equal to 5, the last significant digit is increased by 1. Consider the measurement $207.518 \: \text{m}$. Right now, the measurement contains six significant figures. How would we successively round it to fewer and fewer significant figures? Follow the process as outlined in Table $1$. Table $1$: Rounding examples Number of Significant Figures Rounded Value Reasoning 6 207.518 All digits are significant 5 207.52 8 rounds the 1 up to 2 4 207.5 2 is dropped 3 208 5 rounds the 7 up to 8 2 210 8 is replaced by a 0 and rounds the 0 up to 1 1 200 1 is replaced by a 0 Notice that the more rounding that is done, the less reliable the figure is. An approximate value may be sufficient for some purposes, but scientific work requires a much higher level of detail. Summary • Rounding involves the adjustment of a value to account for the proper number of significant digits. • Rules exist for rounding numbers. Review 1. Why do we round numbers? 2. What do we need to know before we round a number? 3. What is “rounding up”? 4. What is “rounding down”?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.14%3A_Measurement_Uncertainty.txt
How fast do you drive? As you enter the town of Jacinto City, Texas, a sign tells you that the speed limit is 30 miles per hour. But what if you happen to be driving 31 miles an hour? Are you in trouble? Probably not, because there is a certain amount of leeway built into enforcing the regulation. Most speedometers do not measure the vehicle speed very accurately and could easily be off by a mile or so (on the other hand, radar measurements are much more accurate). So, a couple of miles per hour difference won't matter that much. However, if you stretch the limits any further, you may end up receiving a traffic ticket. Significant Figures The significant figures in a measurement consist of all the certain digits in that measurement, plus one uncertain or estimated digit. In the ruler illustration below, the bottom ruler gave a length with 2 significant figures, while the top ruler gave a length with 3 significant figures. In a correctly reported measurement, the final digit is significant but not certain. Insignificant digits are not reported. With either ruler, it would not be possible to report the length at $2.553 \: \text{cm}$, as there is no possible way that the thousandths digit could be estimated. The 3 is not significant and would not be reported. When you look at a reported measurement, it is necessary to be able to count the number of significant figures. The table below details the rules for determining the number of significant figures in a reported measurement. For the examples in the table, assume that the quantities are correctly reported values of a measured quantity. Rule Examples Figure $1$: Significant Figure Rules 1. All nonzero digits in a measurement are significant. • 237 has three significant figures. • 1.897 has four significant figures. 2. Zeros that appear between other nonzero digits are always significant. • 39,004 has five significant figures. • 5.02 has three significant figures. 3. Zeros that appear in front of all of the nonzero digits are called left-end zeros. Left-end zeros are never significant. • 0.008 has one significant figure. • 0.000416 has three significant figures. 4. Zeros that appear after all nonzero digits are called right-end zeros. Right-end zeros in a number that lacks a decimal point are not significant. • 140 has two significant figures. • 75,210 has four significant figures. 5. Right-end zeros in a number with a decimal point are significant. This is true whether the zeros occur before or after the decimal point. • 620.0 has four significant figures. • 19.000 has five significant figures. It needs to be emphasized that to say a certain digit is not significant, does not mean that it is not important or can be left out. Though the zero in a measurement of 140 may not be significant, the value cannot simply be reported as 14. An insignificant zero functions as a placeholder for the decimal point. When numbers are written in scientific notation, this becomes more apparent. The measurement 140 can be written as $1.4 \times 10^2$ with two significant figures in the coefficient. For a number with left-end zeros, such as 0.000416, it can be written as $4.16 \times 10^{-4}$ with 3 significant figures. In some cases, scientific notation is the only way to correctly indicate the correct number of significant figures. In order to report a value of 15,000,000 with four significant figures, it would need to be written as $1.500 \times 10^7$. The right-end zeros after the 5 are significant. The original number of 15,000,000 only has two significant figures. Summary • Significant figures give an indication of the certainty of a measurement. • Rules allow decisions to be made about how many digits to use in any given situation. Review 1. What does a significant figure tell us? 2. What is a left-end zero? 3. What is a right-end zero? 4. What does an insignificant zero do?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.16%3A_Significant_Figures.txt
How old do you think this calculator is? Calculators are great devices. Their invention has allowed for quick computation at work, school, and other places where manipulation of numbers needs to be done rapidly and accurately. But they are only as good as the numbers put into them. The calculator cannot determine how accurate each set of numbers is, and the answer given on the screen must be assessed by the user for reliability. Uncertainty in Addition and Subtraction Consider two separate mass measurements: $16.7 \: \text{g}$ and $5.24 \: \text{g}$. The first mass measurement, $\left( 16.7 \: \text{g} \right)$, is known only to the tenths place, or to one digit after the decimal point. There is no information about its hundredth place and so that digit cannot be assumed to be zero. The second measurement, $\left( 5.24 \: \text{g} \right)$, is known to the hundredths place, or to two digits after the decimal point. When these masses are added together, the result on a calculator is $16.7 + 5.24 = 21.94 \: \text{g}$. Reporting the answer as $21.94 \: \text{g}$ suggests that the sum is known all the way to the hundredths place. However, that cannot be true because the hundredths place of the first mass was completely unknown. The calculated answer needs to be rounded in such a way as to reflect the certainty of each of the measured values that contribute to it. For addition and subtraction problems, the answer should be rounded to the same number of decimal places as the measurement with the least number of decimal places. The sum of the above masses would be properly rounded to a result of $21.9 \: \text{g}$. Example $1$ Determine the combined molecular mass of a glucose molecule and a maltose molecule. Glucose molecule = $180.156\frac{g}{mol}$ Maltose molecule = $342.3\frac{g}{mol}$ Solution $180.156+342.4=522.456$ When adding and subtracting we know to look at the least number of decimals in our starting values; in this case 342.3 has only 1 digit after the decimal, so we need to round our answer to the same place. $522.456\to 522.5\frag{g}{mol} When working with whole numbers, pay attention to the last significant digit that is to the left of the decimal point, and round your answer to that same point. For example, consider the subtraction: \(78,500 \: \text{m} - 362 \: \text{m}$. The calculated result is $78,138 \: \text{m}$. However, the first measurement is known only to the hundreds place, as the 5 is the last significant digit. Rounding the result to that same point means that the correct result is $78,100 \: \text{m}$. Example $2$ What is $4200 + 540$ = ? Solution $4200 + 540 = 4740$ To determine where to round our answer, we look at our starting numbers to see which has the fewest number of decimal places. They both have 0 so we round to the nearest whole number, 4740. Summary • For addition and subtraction problems, the answer should be rounded to the same number of decimal places as the measurement with the least number of decimal places. Review 1. What is the basic principle to use in working with addition and subtraction? 2. What do you pay attention to when working with whole numbers? 3.18: Significant Figures in Multiplication and Division Who should report the numbers - you or your calculator? Calculators do just what is asked of them—no more and no less. However, they sometimes can get a little out of hand. If you multiply 2.48 times 6.3, you get an answer of 15.687, a value that ignores the number of significant figures in either number. Division with a calculator is even worse. When you divide 12.2 by 1.7, the answer you obtain is 7.176470588. Neither piece of data is accurate to nine decimal places, but the calculator does not know that. The human being operating the instrument has to make the decision about how many places to report. Uncertainty in Multiplication and Division The density of a certain object is calculated by dividing the mass by the volume. Suppose that a mass of $37.46 \: \text{g}$ is divided by a volume of $12.7 \: \text{cm}^3$. The result on a calculator would be: $D = \frac{m}{V} = \frac{37.46 \: \text{g}}{12.7 \: \text{cm}^3} = 2.949606299 \: \text{g/cm}^3\nonumber$ The value of the mass measurement has four significant figures, while the value of the volume measurement has only three significant figures. For multiplication and division problems, the answer should be rounded to the same number of significant figures as the measurement with the least number of significant figures. Applying this rule results in a density of $2.95 \: \text{g/cm}^3$, with three significant figures—the same as the volume measurement. Example $1$ Perform the following calculations, rounding the answers to the appropriate number of significant figures. A. $0.048 \: \text{m} \times 32.97 \: \text{m}$ B. $14,570 \: \text{kg} \div 5.81 \: \text{L}$ Step 1: Plan the problem. Analyze each of the measured values to determine how many significant figures should be in the result. Perform the calculation and round appropriately. Apply the correct units to the answer. When multiplying or dividing, the units are also multiplied or divided. Step 2: Calculate. A. $0.048 \: \text{m} \times 32.97 \: \text{m} = 1.6 \: \text{m}^2$ Round to two significant figures because 0.048 has two. B. $14,570 \: \text{kg} \div 5.81 \: \text{L} = 2510 \: \text{kg/L}$ Round to three significant figures because 5.81 has three. Exercise $1$ How many significant figures should the answer contain? $10.61\times 12.133 \times 3.25$ Answer =418; 3 significant figures Summary • For multiplication and division problems, the answer should be rounded to the same number of significant figures as the measurement with the least number of significant figures. Review 1. Perform the calculation and round your answer with the correct number of significant figures. 1. 78.2 ÷ 32 cm3 2. 3.0 m/s × 9.21 s 1. What happens to units in multiplication and division problems?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/03%3A_Measurements/3.17%3A_Significant_Figures_in_Addition_and_Subtraction.txt
What would the philosophers do? People enjoy getting together to discuss things, whether it is how your favorite sports team is doing, what the best new movie is, the current politics, or any number of other topics. Often the question is raised about who is right and who is wrong. If the football game is to be played this coming weekend, all we can do is offer opinions as to its outcome. The game has not been played yet, so we don't know who will actually win. The ancient Greek philosophers did a lot of discussing, with part of their conversations concerning the physical world and its composition. There were different opinions about what made up matter. Some felt one thing was true while others believed another set of ideas. Since these scholars did not have laboratories and had not developed the idea of the experiment, they were left to debate. Whoever could offer the best argument was considered right. However, often the best argument had little to do with reality. One of the on-going debates had to do with sand. The question posed was: into how small of pieces can you divide a grain of sand? The prevailing thought at the time, pushed by Aristotle, was that the grain of sand could be divided indefinitely, that you could always get a smaller particle by dividing a larger one and there was no limit to how small the resulting particle could be. Since Aristotle was such an influential philosopher, very few people disagreed with him. However, there were some philosophers who believed that there was a limit to how small a grain of sand could be divided. One of these philosophers was Democritus (~460-370 B.C.E.), often referred to as the "laughing philosopher" because of his emphasis on cheerfulness. He taught that there were substances called atoms and that these atoms made up all material things. The atoms were unchangeable, indestructible, and always existed. The word "atom" comes from the Greek $\alpha \tau \omicron \mu \omicron \sigma$ and means "indivisible". The atomists of the time (Democritus being one of the leading atomists) believed there were two realities that made up the physical world: atoms and void. There were an infinite number of atoms, but different types of atoms had different sizes and shapes. The void was the empty space in which the atoms moved and collided with one another. When these atoms collided with one another, they might repel each other or they might connect in clusters, held together by tiny hooks and barbs on the surfaces of the atoms. Aristotle disagreed with Democritus and offered his own idea of the composition of matter. According to Aristotle, everything was composed of four elements: earth, air, fire, and water. Deomcritus' theory better explained things, but Aristotle was more influential, so his ideas prevailed. It took almost two thousand years before scientists came around to seeing the atom as Democritus did. How right was Democritus? It is very interesting that Democritus had the basic idea of atoms, even though he had no experimental evidence to support his thinking. We now know more about how atoms hold together in "clusters" (compounds), but the basic concept existed over two thousand years ago. We also know that atoms can be further subdivided, but there is still a lower limit to how small we can break up that grain of sand. Summary • Greek philosophers debated many ideas. • Aristotle and others believed that a grain of sand could be divided indefinitely. • Democritus believed there was a lower limit to the division of a grain of sand. Review 1. How did the ancient Greek philosophers spend their time? 2. What approach did they not have for studying nature? 3. Who was the most influential philosopher of that time? 4. What was the major contribution Democritus made to the thinking of his day? 5. List characteristics of atoms according to Democritus.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.01%3A_Democritus%27_Idea_of_the_Atom.txt
Have you ever lost a screw? The following situation happens all too often. You have taken apart a piece of equipment to clean it up. When you put the equipment back together, somehow you have an extra screw or two. Or you find out that a screw is missing that was a part of the original equipment. In either case, you know something is wrong. You expect to end up with the same amount of material that you started with, not with more or less than what you had originally. Law of Conservation of Mass By the late 1700s, chemists accepted the definition of an element as a substance that cannot be broken down into a simpler substance by ordinary chemical means. It was also clear that elements combine with one another to form more complex substances called compounds. The chemical and physical properties of these compounds are different than the properties of the elements from which they were formed. There were questions about the details of these processes. In the 1790s, a greater emphasis began to be placed on the quantitative analysis of chemical reactions. Accurate and reproducible measurements of the masses of reacting elements and the compounds they form led to the formulation of several basic laws. One of these is called the law of conservation of mass, which states that during a chemical reaction, the total mass of the products must be equal to the total mass of the reactants. In other words, mass cannot be created or destroyed during a chemical reaction, but is always conserved. As an example, consider the reaction between silver nitrate and sodium chloride. These two compounds will dissolve in water to form silver chloride and sodium nitrate. The silver chloride does not dissolve in water, so it forms a solid that we can filter off. When we evaporate the water, we can recover the sodium nitrate formed. If we react 58.5 grams of sodium chloride with 169.9 grams of silver nitrate, we start with 228.4 grams of materials. After the reaction is complete and the materials separated, we find that we have formed 143.4 grams of silver chloride and 85.0 grams of sodium nitrate, giving us a total mass of 228.4 grams for the products. So, the total mass of reactants equals the total mass of products, a proof of the law of conservation of mass. Summary • The law of conservation of mass states that, during a chemical reaction, the total mass of the products must be equal to the total mass of the reactants. Review 1. The law of conservation of mass states that, during a chemical reaction, the total ______ of the products must be equal to the total ______ of the reactants. 2. Describe an example of the law of conservation of mass. 4.03: Law of Multiple Proportions What are the similarities and differences between a unicycle and a bicycle? Just from the words themselves, the astute Latin-speaking scholar can tell that, whatever it is made of, the unicycle has one of them (uni = "one") and the bicycle has two (bi = "two"). From pictures, we get additional information that helps us tell the two apart. The unicycle has one wheel and the bicycle has two. In particular, they are made up of the same materials, and the only significant difference is the number of wheels on the two vehicles. Now—how many wheels are on a tricycle? Law of Multiple Proportions Once the idea that elements combined in definite proportions to form compounds was established, experiments also began to demonstrate that the same pairs of certain elements could combine to form more than one compound. Consider the elements carbon and oxygen. Combined in one way, they form the familiar compound carbon dioxide. In every sample of carbon dioxide, there are $32.0 \: \text{g}$ of oxygen present for every $12.0 \: \text{g}$ of carbon. By dividing $32.0$ by $12.0$, this simplifies to a mass ratio of oxygen to carbon of 2.66 to 1. There is another compound that forms from the combination of carbon and oxygen called carbon monoxide. Every sample of carbon monoxide contains $16.0 \: \text{g}$ of oxygen for every $12.0 \: \text{g}$ of carbon. This is a mass ratio of oxygen to carbon of 1.33 to 1. In the carbon dioxide, there is exactly twice as much oxygen present as there is in the carbon monoxide. This example illustrates the law of multiple proportions: whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers. In carbon monoxide, on the left, there is $1.333 \: \text{g}$ of oxygen for every $1 \: \text{g}$ of carbon. In carbon dioxide, on the right, there is $2.666 \: \text{g}$ of oxygen for every gram of carbon. So the ratio of oxygen in the two compounds is 1:2, a small whole number ratio. The difference between carbon monoxide and carbon dioxide is significant. Carbon monoxide is a deadly gas, formed from the incomplete combustion of some carbon-containing materials (such as wood and gasoline). This compound will attach to hemoglobin in the red blood cells and block the binding of oxygen to those cells. If oxygen does not bind, it cannot be carried to the cells of the body where it is needed, and death can occur. Carbon dioxide, on the other hand, is not toxic like carbon monoxide is. However, it can displace oxygen in systems since it is heavier. Carbon dioxide fire extinguishers cut off the flow of oxygen in a fire, putting out the fire. Summary • The law of multiple proportions states that whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers. Review 1. State the law of multiple proportions. 2. In carbon dioxide (CO2), how many grams of oxygen (O) would there be if there are 24 grams of carbon (C)? 3. How many grams of carbon (C) would be present in carbon monoxide (CO) that contains 2.666 grams of oxygen (O)?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.02%3A_Law_of_Conservation_of_Mass.txt
We use electricity for many purposes—from cooking, to powering our televisions, to charging our cell phones. Wherever we travel in the United States, we want electricity to be available. What we also want (although we usually don't think about it) is for the electricity supply to be the same wherever we go. We want the same voltage (110 volts for the U.S.) to come from the outlet to whatever we plug in. If the voltage is less, the system will not work. If it is more, the equipment will be damaged. We want a definite amount of voltage—no more and no less. Law of Definite Proportions The discovery that mass was always conserved in chemical reactions was soon followed by the law of definite proportions, which states that a given chemical compound always contains the same elements in the exact same proportions by mass. As an example, any sample of pure water contains $11.19\%$ hydrogen and $88.81\%$ oxygen by mass. It does not matter where the sample of water came from or how it was prepared. Its composition, like that of every other compound, is fixed. Another example is carbon dioxide. This gas is produced from a variety of reactions, often by the burning of materials. The structure of the gas consists of one atom of carbon and two atoms of oxygen. Carbon dioxide production is of interest in many areas, from the amount we breathe to the amount of the gas produced by burning wood or fossil fuels. By knowing the exact composition of carbon dioxide, we can make predictions as to the effects of different chemical processes. Summary • The law of definite proportions states that a given chemical compound always contains the same elements in the exact same proportions by mass. Review 1. State the law of definite proportions. 2. Will the composition of water molecules vary depending on its source? 3. Why is this law important? Explore More Use the resource below to answer the questions that follow. 1. How many hydrogen atoms are there in a molecule of water? 2. How many oxygen atoms are there in a molecule of water? 3. What is the mass ratio of hydrogen to oxygen in a molecule of water? 4. Will the mass ratio of hydrogen to oxygen change depending on the size of the sample? For example if the sample size was 10 L or 10,000 L. 4.05: Mass Ratio Calculation What are the similarities and differences between these two equations? One fundamental law of chemistry deals with the fact that we cannot create or destroy matter (using chemical means). When a reaction is run, the number of atoms of each specific type must be the same on both sides of the equation. For some materials, it turns out that one element can combine with a second element in more than one ratio. Carrying out mass ratio calculations helped establish the law of multiple proportions. Mass Ratio Calculations Example $1$ Copper reacts with chlorine to form two compounds. Compound A consists of $4.08 \: \text{g}$ of copper for every $2.28 \: \text{g}$ of chlorine. Compound B consists of $7.53 \: \text{g}$ of copper for every $8.40 \: \text{g}$ of chlorine. What is the lowest whole number mass ratio of copper that combines with a given mass of chlorine? Known • Compound A $= 4.08 \: \text{g} \: \ce{Cu}$ and $2.28 \: \text{g} \: \ce{Cl}$ • Compound B $= 7.53 \: \text{g} \: \ce{Cu}$ and $8.40 \: \text{g} \: \ce{Cl}$ Apply the law of multiple proportions to the two compounds. For each compound, find the grams of copper that combine with $1.00 \: \text{g}$ of chlorine by dividing the mass of copper by the mass of chlorine. Then find the ratio of the masses of copper in the two compounds by dividing the larger value by the smaller value. Step 2: Calculate $\text{Compound A} \: \frac{4.08 \: \text{g} \: \ce{Cu}}{2.28 \: \text{g} \: \ce{Cl}} = \frac{1.79 \: \text{g} \: \ce{Cu}}{1.00 \: \text{g} \: \ce{Cl}}\nonumber$ $\text{Compound B} \: \frac{7.53 \: \text{g} \: \ce{Cu}}{8.40 \: \text{g} \: \ce{Cl}} = \frac{0.896 \: \text{g} \: \ce{Cu}}{1.00 \: \text{g} \: \ce{Cl}}\nonumber$ Compare the masses of copper per gram of chlorine in the two samples. $\frac{1.79 \: \text{g} \: \ce{Cu} \: \text{(in compound A)}}{0.896 \: \text{g} \: \ce{Cu} \: \text{(in compound B)}} = \frac{2.00}{1} = 2:1\nonumber$ The mass ratio of copper per gram of chlorine in the two compounds is 2:1. Step 3: Think about your result. The ratio is a small whole-number ratio. For a given mass of chlorine, compound A contains twice the mass of copper as does compound B. Summary • The mass ratio gives the mass of an element that is found in combination with another element. Review 1. What does the mass ratio between two elements tell us? 2. If we compare the mass ratio of elements in one compound to that in an second compound what can we learn? Give an example from the lesson above. 3. In compound A, there is 6.3 g of hydrogen and 18.7 g of carbon, while in compound B there is 6.9 g of hydrogen and 41.0 g of carbon, what is the carbon to hydrogen mass ratio in each compound and how do these ratios compare? 4. What are lowest ratios of hydrogen and carbon in compounds A and B? Predict their formulas	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.04%3A_Law_of_Definite_Proportions.txt
"Pick a little, talk a little, pick a little, talk a little, Cheep cheep cheep, talk a lot, pick a little more" These lyrics from the musical "Music Man" sum up the way science was done for centuries. The lyrics refer to a group of gossiping ladies, but the outcome was the same. The Greek and Roman philosophers debated, discussed, and sometimes even attacked one another. But the mode of discovery was talk. There was no experimentation—the idea had not yet been thought of. So science did not develop very far, and there was no reliable way to establish what was true and what was false. John Dalton While it must be assumed that many more scientists, philosophers, and others studied the composition of matter after Democritus, a major leap forward in our understanding of the composition of matter took place in the 1800s with the work of the British scientist John Dalton. He started teaching school at age twelve, and was primarily known as a teacher. In his twenties, he moved to the growing city of Manchester, where he was able to pursue some scientific studies. His work in several areas of science brought him a number of honors. When he died, over 40,000 people in Manchester marched at his funeral. Dalton studied the weights of various elements and compounds. He noticed that matter always combined in fixed ratios based on weight, or volume in the case of gases. Chemical compounds always contain the same proportion of elements by mass, regardless of amount, which provided further support for Proust's law of definite proportions. Dalton also observed that there could be more than one combination of two elements. Dalton's Atomic Theory (1804) From his own experiments and observations, as well as the work of his peers, Dalton proposed a new theory of the atom. This later became known as Dalton's atomic theory. The general tenets of this theory are as follows: • All matter is composed of extremely small particles called atoms. • Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties. • Atoms cannot be subdivided, created, or destroyed. • Atoms of different elements can combine in simple whole number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged. Dalton's atomic theory has been largely accepted by the scientific community, with the exception of three changes. We now know that (1) an atom can be further subdivided, (2) all atoms of an element are not identical in mass, and (3) using nuclear fission and fusion techniques, we can create or destroy atoms by changing them into other atoms. Summary • Dalton proposed his atomic theory in 1804. • The general tenets of this theory are: • All matter is composed of extremely small particles called atoms • Atoms cannot be subdivided, created, or destroyed. • Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties. • Atoms of different elements can combine in whole number ratios to form chemical compounds; • In chemical reaction, atoms are rearranged, combined, or separated. Review 1. Did Dalton believe that atoms could be created or destroyed? 2. According to Dalton's theory, all atoms of the same ______ are identical in size, mass, and other properties. 3. What parts of the theory are not considered valid any more?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.06%3A_Dalton%27s_Atomic_Theory.txt
What could this hilly blue surface possibly be? Do you have any idea? The answer is a single atom of the element cobalt. The picture was created using a scanning tunneling microscope. No other microscope can make images of things as small as atoms. How small are atoms? You will find out in this lesson. What Are Atoms? Atoms are the building blocks of matter. They are the smallest particles of an element that still have the element's properties. Elements, in turn, are pure substances—such as nickel, hydrogen, and helium—that make up all kinds of matter. All the atoms of a given element are identical in that they have the same number of protons, one of the building blocks of atoms (see below). They are also different from the atoms of all other elements, as atoms of different elements have different number of protons. Size of Atoms Unlike bricks, atoms are extremely small. The radius of an atom is well under 1 nanometer, which is one-billionth of a meter. If a size that small is hard to imagine, consider this: trillions of atoms would fit inside the period at the end of this sentence. Although all atoms are very small, elements vary in the size of their atoms. The figure below compares the sizes of atoms of more than 40 different elements. The elements in the figure are represented by chemical symbols, such as H for hydrogen and He for helium. Of course, real atoms are much smaller than the circles representing them in the figure below. Q: Which element in the figure above has the biggest atoms? A: The element in the figure with the biggest atoms is cesium (Cs). Subatomic Particles Although atoms are very tiny, they consist of even smaller particles. Three main types of particles that make up all atoms are: • protons, which have a positive electric charge. • electrons, which have a negative electric charge. • neutrons, which are neutral in electric charge. The model in the figure below shows how these particles are arranged in an atom. The particular atom represented by the model is helium, but the particles of all atoms are arranged in the same way. At the center of the atom is a dense area called the nucleus, where all the protons and neutrons are clustered closely together. The electrons constantly move around the nucleus. Helium has two protons and two neutrons in its nucleus and two electrons moving around the nucleus. Atoms of other elements have different numbers of subatomic particles, but the number of protons always equals the number of electrons. This makes atoms neutral in charge because the positive and negative charges "cancel out." Q: Lithium has three protons, four neutrons, and three electrons. Sketch a model of a lithium atom, similar to the model above for helium. A: Does your sketch resemble the model in the figure below? The model has three protons (blue) and four neutrons (gray) in the nucleus, with three electrons (red) moving around the nucleus. Q: All atoms of carbon have six protons. How many electrons do carbon atoms have? A: Carbon atoms must have six electrons to "cancel out" the positive charges of the six protons. That's because atoms are always neutral in electric charge. Summary • Atoms are the building blocks of matter. They are the smallest particles of an element that still have the element's properties. • All atoms are very small, but atoms of different elements vary in size. • Three main types of particles that make up all atoms are protons, neutrons, and electrons. Review 1. What is an atom? 2. Which of the following statement(s) are true about the atoms of any element? 1. The number of protons in an atom of an element is unique to each element. 2. The number of protons and neutrons in an atom of an element is unique to each element 3. A proton in an atom of one element is identical to a proton in an atom of another element. 4. The number of protons in an atom of an element is the same for all elements. 3. Which of the following statements explains why atoms are always neutral in charge 1. They have the same number of protons as the atoms of all other elements. 2. They have protons that are identical to the protons of all other elements. 3. They have the same size as the atoms of all other elements. 4. They have the same number of protons as electrons.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.07%3A_Atom.txt
What causes a power outage? In a power outage, all your electrical equipment suddenly stops working. The radio was on just a minute ago and now it is silent. What happened? Somewhere between a power generator and your electrical device was an interruption. Power stopped flowing through the wires and into your radio. That “power” turns out to be electrons that move through the wires and cause an electrical current to flow. Is There Anything Inside an Atom? As the nineteenth century began to draw to a close, the concept of atoms was well-established. We could determine the mass of different atoms and had some good ideas about the atomic composition of many compounds. Dalton’s atomic theory held that atoms were indivisible, so scientists did not ask questions about what was inside the atom – it was solid and could not be broken down further. But then things began to change. The Electron In 1897, English physicist J.J. Thomson (1856-1940) experimented with a device called a cathode ray tube, in which an electric current was passed through gases at low pressure. A cathode ray tube consists of a sealed glass tube fitted at both ends with metal disks called electrodes. The electrodes are then connected to a source of electricity. One electrode, called the anode, becomes positively charged while the other electrode, called the cathode, becomes negatively charged. A glowing beam (the cathode ray) travels from the cathode to the anode. Earlier investigations by Sir William Crookes and others had been carried out to determine the nature of the cathode ray. Thomson modified and extended these experiments in an effort to learn about these mysterious rays. He discovered two things, which supported the hypothesis that the cathode ray consisted of a stream of particles. • When an object was placed between the cathode and the opposite end of the tube, it cast a shadow on the glass. • A cathode ray tube was constructed with a small metal rail between the two electrodes. Attached to the rail was a paddle wheel capable of rotating along the rail. Upon starting up the cathode ray tube, the wheel rotated from the cathode towards the anode. This proved that the cathode ray was made of particles which must have mass. Crooke had first observed this phenomenon and attributed it to pressure by these particles on the wheel. Thomson correctly surmised that these particles were producing heat, which caused the wheel to turn. In order to determine if the cathode ray consisted of charged particles, Thomson used magnets and charged plates to deflect the cathode ray. He observed that cathode rays were deflected by a magnetic field in the same manner as a wire carrying an electric current, which was known to be negatively charged. In addition, the cathode ray was deflected away from a negatively charged metal plate and towards a positively charged plate. Thomson knew that opposite charges attract one another, while like charges repel one another. Together, the results of the cathode ray tube experiments showed that cathode rays are actually streams of tiny negatively charged particles moving at very high speeds. While Thomson originally called these particles corpuscles, they were later named electrons. Thomson conducted further experiments, which allowed him to calculate the charge-to-mass ratio $\frac{e}{m_e}$ of the electron. In units of coulombs to grams, this value is 1.8 × 108 Coulombs/gram. He found that this value was a constant and did not depend on the gas used in the cathode ray tube or on the metal used as the electrodes. He concluded that electrons were negatively charged subatomic particles present in atoms of all elements. Summary • Cathode rays are deflected by a magnetic field. • The rays are deflected away from a negatively charged electrical field and toward a positively charge field. • The charge/mass ratio for the electron is 1.8 × 108 Coulombs/gram. Review 1. What subatomic particle creates electric power, and how does it do it? 2. Whose work did Thomson repeat and revise? 3. What experiment did Thomson perform that showed cathode rays to be particles? 4. How did he show that these particles had a charge on them? 5. Did the cathode ray have positive or negative charge?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.08%3A_Electrons.txt
Can you name this car? Describing what we can see is a fairly easy matter. If we are asked to describe the sports car illustrated below, we could all quickly come up with a fairly accurate description. A person knowledgeable about cars would include more details, but everyone would have the basic information in their description. What makes the description easy to come up with? We can see it, we have a common language to describe it (size, color, construction), and we have a basic idea of what it is (a car, not a house or a tree). Scientists have far more difficulty in describing things they cannot see. There is no way to look directly at an atom and see its detailed structure. When a discovery is first made, there is often no language to use to tell others exactly what it is. This was the problem in talking about the atom and its structure. Putting the Puzzle Pieces Together Research builds upon itself – one piece connects to another. Sometimes the puzzle doesn't seem to make sense because some of the pieces are missing at the moment. Each finding gives a clearer picture of the whole and also raises new questions. The detective work that led to the discovery of the proton was built upon finding pieces to the puzzle and putting them together in the right way. The electron was discovered using a cathode ray tube. An electric current was passed from the cathode (the negative pole) to the anode (positive pole). Several experiments showed that particles were emitted at the cathode and that these particles had a negative charge. These experiments demonstrated the presence of electrons. If cathode rays are electrons that are given off by the metal atoms of the cathode, then what remains of the atoms that have lost those electrons? We know several basic things about electrical charges. They are carried by particles of matter. Millikan's experiment showed that they exist as whole-number multiples of a single basic unit. Atoms have no overall electrical charge, meaning that each and every atom contains an exactly equal number of positively and negatively charged particles. A hydrogen atom is the simplest kind of atom with only one electron. When that electron is removed, a positively charged particle should remain. Discovery of the Proton In 1886, Eugene Goldstein (1850-1930) discovered evidence for the existence of this positively charged particle. Using a cathode ray tube with holes in the cathode, he noticed that there were rays traveling in the opposite direction from the cathode rays. He called these canal rays and showed that they were composed of positively charged particles. The proton is the positively charged subatomic particle present in all atoms. The mass of the proton is about 1840 times the mass of the electron. Summary • When an electron is removed from a hydrogen atom, a proton remains. • Goldstein observed rays traveling in the opposite direction of the cathode rays in a cathode ray tube. • He demonstrated that these rays were positive particles and called the canal rays. Review 1. Why is it easy to describe things we can see? 2. Why did researchers believe that the particle left after electrons were emitted as cathode rays had to be positive? 3. Atoms, which are always neutral in electric charge, contain electrons as well as protons and neutrons. An electron has an electrical charge of -1. If an atom has three electrons, infer how many protons it has. 4. How many electrons does it take to weight the same as one proton?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.09%3A_Protons.txt
Was Sherlock Holmes real or the product of someone's imagination? The most famous detective in history and literature never existed. Sherlock Holmes was the creation of the British author Sir Arthur Conan Doyle. This mythical person had capabilities far beyond those of mere mortals. Holmes was capable of spotting the tiniest clue, the smallest piece of evidence to solve the crime. He could link all sorts of seemingly irrelevant data into a coherent whole to clear up whatever mystery he was dealing with. The Quest for the Neutron Clues are generally considered to involve the presence of something – a footprint, a piece of fabric, a bloodstain, something tangible that we can measure directly. The discoveries of the electron and the proton were accomplished with the help of those kinds of clues. Cathode ray tube experiments showed both the negatively charged electrons emitted by the cathode and the positively charged proton (also emitted by the cathode). The neutron was initially found not by a direct observation, but by noting what was not found. Research had shown the properties of the electron and the proton. Scientists learned that approximately 1,837 electrons weighed the same as one proton. There was evidence to suggest that electrons went around the heavy nucleus composed of protons. Charge was balanced with equal numbers of electrons and protons which made up an electrically neutral atom. But there was a problem with this model – the atomic number (number of protons) did not match the atomic weight. In fact, the atomic number was usually about half the atomic weight. This indicated that something else must be present. That something must weigh about the same as a proton, but could not have a charge – this new particle had to be electrically neutral. In 1920, Ernest Rutherford tried to explain this phenomenon through the presence of another particle in the nucleus. He proposed that the "extra" particles were protons and electrons that had combined into a new particle in the nucleus (this did not turn out to be the case). These new particles would have a mass very similar to a proton, but would be electrically neutral since the positive charge of the proton and the negative charge of the electron would cancel each other out. In 1930, German researchers bombarded the element beryllium with alpha particles (helium nuclei containing two protons and two neutrons with a charge of +2). The particles produced in this process had strong penetrating power, which suggested they were fairly large. In addition, they were not affected by a magnetic field, so they were electrically neutral. The French husband-wife research team of Frederic and Irene Joliot-Curie used these new "rays" to bombard paraffin, which was rich in protons. The unknown particles produced a large emission of protons from the paraffin. The English physicist James Chadwick (1891-1974) repeated these experiments and studied the energy of these particles. By measuring velocities, he was able to show that the new particle has essentially the same mass as a proton. So we now have a third subatomic particle with a mass equal to that of a proton, but with no charge. This particle is called the neutron. Chadwick won the Nobel Prize in Physics in 1935 for his research. Neutron Applications Neutrons can be used in a variety of ways. One important use is in nuclear fission to produce new isotopes. A neutron will collide with a large atom (such as uranium) and cause it to split into smaller atoms, such as in figure below. Nuclear reactors utilize chain reactions involving neutrons to heat water which drive turbines for the generation of electricity. When a neutron collides with a large atom, the atom splits with the release of more neutrons and also a large amount of energy. The energy converts water to steam for the turbine, while the neutrons serve to continue the chain reaction (see figure below). Summary • Rutherford proposed that "extra" particles in nucleus were combinations of protons and electrons. • Bombardment of beryllium with alpha particles produced large, neutral particles. • Chadwick determined the mass of the neutron. • Nuclear fission produces new elements. • Nuclear reactors use chain reactions to produce heat. Review 1. How did Rutherford try to explain the differences between the number of protons in the nucleus and the atomic weight? 2. What did German researchers find when they bombarded beryllium with alpha particles? 3. What did Chadwick determine about these new particle (observed by the German scientist and the Curies)?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.10%3A_Neutrons.txt
How old do you think this TV is? The TV set seen above is becoming harder and harder to find these days. The main reason is because they are older and based on outdated technology. The new TV sets are flat screen technology that take up less space and give better picture quality, especially with the advent of high-definition broadcasting. The technology used in the older TV sets used cathode ray tubes. A beam of electrons was sprayed to a picture tube which was treated to react with the electrons to produce an image. Similar CRT devices were used in computer monitors, now also replaced by flat screen monitors. Discovery of the Electron The first discovery of a subatomic particle was a result of experiments into the nature of the relationship between electricity and matter. Cathode Rays The first cathode ray tube prototype was developed by Heinrich Geissler, a German glassblower and physicist. He used a mercury pump to create a vacuum in a tube. Geissler explored a number of techniques to remove air from the tube and to prevent leaks, as well as ways to get good connections of the wires in the tubes. In 1878, Sir William Crookes, a British scientist, displayed the first cathode rays using a modification of the Geissler apparatus. His major contribution to construction of the tube was to develop ways to evacuate almost all the air from the tube. Crookes also carried out many experiments using more reliable equipment to confirm earlier findings about the properties of cathode rays. He made two discoveries which supported the hypothesis that the cathode ray consisted of a stream of particles: • When an object was placed between the cathode and the opposite end of the tube, it cast a shadow on the glass. The shadow caused by the object indicates that particles were being blocked on their way from the cathode to the anode. • A cathode ray tube was constructed with a small metal rail between the two electrodes. Attached to the rail was a paddle wheel capable of rotating along the rail. Upon starting up the cathode ray tube, the wheel rotated from the cathode towards the anode. Notice that the cathode and anode are positioned so that the rays will strike the top of the paddle wheel. Crookes concluded that the cathode ray was made of particles which must have mass. Further Research with the Crookes Tube Crookes' work opened the door to a number of important discoveries. Other scientists were able to demonstrate that the "cathode ray" was actually a stream of electrons. In 1897, Karl Ferdinand Braun developed the first oscilloscope, using a cathode ray tube to see an electrical pulse as it passed through the instrument. The invention of television would not have been possible without the cathode ray tube. Work with a modified system led to the discovery of x-rays in 1895 by the German physicist Wilhelm Roentgen. This simple device has led to major advances in science and technology. Summary • The cathode ray tube was first invented by Sir William Crookes. • Experiments showed that the rays had mass. Review • Who developed the first cathode ray tube? • What improvement did Crookes make to the cathode ray tube? • How did Crookes show there were particles being emitted? • What did Karl Ferdinand Braun invent? • What did Wilhelm Roentgen invent?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.11%3A_Cathode_Ray_Tube.txt
"How tall are you? How much do you weigh?" Questions like these are easy to answer because we have tools to make the measurements. A yard stick or tape measure will suffice to measure height. You can stand on a bathroom scale and determine your weight. It is a very different matter to measure properties of objects that we cannot see with the naked eye. If we want to measure the size of a germ, we have to use a microscope. To learn the size of a single molecule, we have to use even more sophisticated instruments. So how would we measure something even smaller than a molecule, even smaller than an atom? Charge and Mass of the Electron The man who measured properties of the electron was Robert Millikan (1868-1953). He taught himself physics while a student at Oberlin College' since there was nobody on the faculty to instruct him in this field. Millikan completed postgraduate research training in the U.S. and in Germany. His studies on the properties of the electron proved to be of great value in many areas of physics and chemistry. Oil Drop Experiment Millikan carried out a series of experiments between 1908 and 1917 that allowed him to determine the charge of a single electron, famously known as the oil drop experiment. Millikan sprayed tiny drops of oil into a chamber. In his first experiment, he simply measured how fast the drops fell under the force of gravity. He could then calculate the mass of the individual drops. Then, he sprayed oil drops and applied an electrical charge to them by shining x-rays up through the bottom of the apparatus. The x-rays ionized the air, causing electrons to attach to the oil drops. The oil drops picked up static charge and were suspended between two charged plates. Millikan was able to observe the motion of the oil drops with a microscope and found that the drops lined up in a specific way between the plates, based on the number of electric charges they had acquired. Millikan used the information to calculate the charge of an electron. He determined the charge to be $1.5924 \times 10^{-19} \: \text{C}$, where $\text{C}$ stands for coulomb, which is one ampere*second. Today, the accepted value of the charge of an electron is $1.602176487 \times 10^{-19} \: \text{C}$. Millikan's experimental value proved very accurate; it is within $1\%$ of the currently accepted value. Millikan later used the information from his oil drop experiment to calculate the mass of an electron. The accepted value today is $9.10938215 \times 10^{-31} \: \text{kg}$. The incredibly small mass of the electron was found to be approximately 1/1840 the mass of a hydrogen atom. Therefore, Scientists realized that atoms must contain another particle that carries a positive charge and is far more massive than the electron. Summary • The oil drop experiment allowed Millikan to determine the charge on the electron. • He later used this data to determine the mass of the electron. Review • How did Millikan learn physics in college? • What did Millikan use to pick up static charge? • Where did the oil drops go to be measured?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.12%3A_Oil_Drop_Experiment.txt
What is this model airplane composed of? Millions of children over the years have enjoyed building models - this model airplane is one example of the types of models that can be constructed. Perhaps sixty years ago the models were made of balsa wood, a very light material. Parts would be cut by hand, carefully glued together, and then covered with paper or other fabric. The development of plastics made the construction of model aircraft much simpler in many respects. And, the end-product is more durable and damage-proof. A model serves a useful purpose – it gives us an idea of what the real thing is like. The model plane seen above has wings, a tail, and an engine just like the real thing. This model also has a propeller, as is the case with most small planes and some smaller passenger planes. However, the model is not the real thing. We certainly cannot fly people or cargo in the model (besides maybe a tiny mouse), but we can get some idea of what a real plane looks like and how it works. Science uses many models to explain ideas. We model the electron as a very small particle with a negative charge. That gives us a picture, but a very incomplete one. This picture works fine for most chemists, but is inadequate for a physicist. Models give us a start toward understanding structures and processes, but certainly are not a complete representation of the entity we are examining. Atomic Models The electron was discovered by J.J. Thomson in 1897. The existence of protons was also known, as was the fact that atoms were neutral in charge. Since the intact atom had no net charge and the electron and proton had opposite charges, the next step after the discovery of subatomic particles was to figure out how these particles were arranged in the atom. This is a difficult task because of the incredibly small size of the atom. Therefore, scientists set out to design a model of what they believed the atom could look like. The goal of each atomic model was to accurately represent all of the experimental evidence about atoms in the simplest way possible. Following the discovery of the electron, J.J. Thomson developed what became known as the "plum pudding" model in 1904. Plum pudding is an English dessert similar to a blueberry muffin. In Thomson's plum pudding model of the atom, the electrons were embedded in a uniform sphere of positive charge, like blueberries stuck into a muffin. The positive matter was thought to be jelly-like, or similar to a thick soup. The electrons were considered somewhat mobile. As they got closer to the outer portion of the atom, the positive charge in the region was greater than the neighboring negative charges, and the electron would be pulled back toward the center region of the atom. However, this model of the atom soon gave way to a new model developed by New Zealander Ernest Rutherford (1871-1937) about five years later. Thomson did still receive many honors during his lifetime, including being awarded the Nobel Prize in Physics in 1906 and a knighthood in 1908. Summary • A model gives an idea of what something looks like, but is not the real thing. • The "plum pudding" model of the atom consisted of a uniform sphere of positive charge with negative electrons embedded in the sphere. Review • What is a model? • Why are models useful in science? • In Thomson's model of the atom, where were the electrons? • What was the positive charge in this model? • What kept the electrons in the atom? • Whose model replaced Thomson's? • What awards did Thomson receive?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.13%3A_Plum_Pudding_Atomic_Model.txt
How much space do bricks occupy? As we look at the world around us, it looks pretty solid. We hit a wall with our hand and the hand stops – it does not (normally) go through the wall. We think of matter as occupying space. But there is a lot of empty space in matter. In fact, most of the matter is empty space. The Gold Foil Experiment In 1911, Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experiments that would completely change the accepted model of the atom. They bombarded very thin sheets of gold foil with fast moving alpha particles. Alpha particles, a type of natural radioactive particle, are positively charged particles with a mass about four times that of a hydrogen atom. According to the accepted atomic model—in which an atom's mass and charge are uniformly distributed throughout the atom—the scientists hypothesized that all of the alpha particles would pass through the gold foil with only a slight deflection, or none at all. Surprisingly, while most of the alpha particles were indeed not deflected, a very small percentage (about 1 in 8000 particles) bounced off the gold foil at very large angles. Some were even redirected back toward the source. No prior knowledge had prepared them for this discovery. In a famous quote, Rutherford exclaimed that it was "as if you had fired a 15-inch [artillery] shell at a piece of tissue and it came back and hit you." Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom's interior, which he called the nucleus. The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons. Rutherford's atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. The electrons are distributed around the nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the nucleus would be about the size of a marble. Rutherford's model proved to be an important step towards a full understanding of the atom. However, it did not completely address the nature of the electrons and the way in which they occupied the vast space around the nucleus. It was not until some years later that a full understanding of the electron was achieved. This proved to be the key to understanding the chemical properties of elements. Summary • Bombardment of gold foil with alpha particles showed that a very small percentage of alpha particles were deflected. • The nuclear model of the atom consists of a small and dense positively charged interior surrounded by a cloud of electrons. Review 1. What is an alpha particle? 2. What did Rutherford observe from shooting thousands and thousands of alpha particles at a thin piece of gold foil? 3. How did Rutherford explain the observation that most alpha particles went straight through the gold foil? 4. What did he say about the particles that were deflected? 5. Describe Rutherford’s nuclear model.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.14%3A_Gold_Foil_Experiment.txt
How is science like a jigsaw puzzle? Many people enjoy putting jigsaw puzzles together. As the different pieces go together, the picture begins to become clearer. When the puzzle is completed, you see that what had been a confused collection of individual components that made little or no sense by themselves fit together to give a clear picture. Science works the same way as a jigsaw puzzle. Different researchers make individual discoveries that answer a specific question or questions. When enough data are gathered, we get a better understanding of a process or a structure. The experiments of Crooke, Millikan, Rutherford, and many others gave us pieces of the puzzle that is the atomic nucleus. Different Models of the Nucleus After the electron and proton were discovered, people began to try to build a picture of the atom. The Thomson model had electrons mixed in with a positive core of some sort that provided electrical neutrality. Rutherford showed that this model was incomplete. His picture of the atom involved a small solid core that alpha particles could zoom past with very few collisions. So the picture of the atom became a little clearer – electrons surrounded a very small core nucleus. The discovery of the neutron helped fill out the picture even more. We now have protons and neutrons in a concentrated center of the atom with electrons surrounding the nucleus. One problem still existed. We have a number of positively charged protons in the nucleus. Why don't they push each other apart? Physicists postulate a strong nuclear force that acts at very short distances. At these distances the attraction between protons is greater than the force causing the positive charges to push each other away. Neutrons are also involved in this process somehow. So the strong force holds protons together, it holds neutrons together, and it causes protons and neutrons to be attracted to one another. Summary • Rutherford proposed a model of the atomic nucleus which had a solid core. • Physicists postulate a strong nuclear force holds the protons and neutrons together in the nucleus of the atom. Review 1. How did Rutherford change our thinking about atomic structure? 2. What is our current picture of the atom? 3. Why is the presence of positively charged protons a problem with current models of the atom? 4. How do we explain why the nucleus does not fall apart? 4.16: Atomic Number What is unique about each one of us? For the vast majority of people, it is not their name, because it is quite possible for others in the world to have the same name (check it out by doing an internet search for your name and see how many other of "you" there are). It is not your physical description. Eye-witnesses to crime scenes often pick the wrong person when trying to identify the criminal. There may be some unique identifiers for us. If you have a cell phone in your name, nobody else in the world has that number. Email addresses are different for each of us, which is a good thing since we can email almost anywhere in the world. Our DNA is unique, but getting a DNA analysis is expensive and time-consuming, so we really don't want to have to explore that. Organizing the Elements One of the goals of science is to discover the order in the universe and to organize information that reflects that order. As information about the different elements was made known, efforts were made to see if there were patterns in all of the data. An early attempt to organize data was made by Mendeleev, who developed the first periodic table. His data set was based on atomic weights and was instrumental in providing clues as to the possible identity of new elements. Once we learned the details of the atomic nucleus, the table was based on the number of protons in the nucleus, called the atomic number of the element. Atomic Number The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. This means that the number of protons is the characteristic which makes each element unique compared to all other elements. Elements are different because of their atomic number. The periodic table displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Following on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, and so forth. Since atoms are neutral, the number of electrons is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Manganese (atomic number 25) would have twenty-five protons and twenty-five electrons. The classification of elements by atomic number allows us to understand many properties of the atom and makes it possible to predict behaviors instead of just having to memorize everything. Summary • The atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element • The number of electrons is equal to the number of protons in an atom of an element. Review 1. What is the atomic number of an atom? Why is this number important? 2. Using a periodic table, what is the atomic number of helium have 3. How many protons are in the following elements:? 1. Ne 2. Ca 3. Pt 4. Write the symbol for the element with the following atomic number: 1. 18 2. 41 3. 82 4. 12	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.15%3A_Atomic_Nucleus.txt
How can you determine the mass of a chemical? Often a student will need to weigh out a chemical for an experiment. If he or she uses a watch glass (a small, round piece that will hold the solid chemical), the weight of the watch glass must be determined first. Then the solid is added to the glass and the weight of the glass plus the solid is measured. The balance reading will be the total of the glass plus the chemical. History of Atomic Weight Determinations As a part of his research on atoms, John Dalton determined a number of atomic weights of elements in the early 1800s. Atomic weights were the basis for the periodic table that Mendeleev developed. Originally all atomic weights were based on a comparison to hydrogen, which has an atomic weight of one. After the discovery of the proton, scientists assumed that the weight of an atom was essentially that of the protons—electrons were known to contribute almost nothing to the atomic weight of the element. This approach worked until we learned how to determine the number of protons in an element. We then saw that the atomic weight for an element was often twice the number of protons (or more). The discovery of the neutron provided the missing part of the picture. The atomic mass is now known to be the sum of the protons and neutrons in the nucleus. Mass Number Rutherford showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number is defined as the total number of protons and neutrons in an atom. It can be calculated by adding the number of neutrons and the number of protons (atomic number) together. Mass number = atomic number + number of neutrons Consider Table $1$ below that shows data from the first six elements of the periodic table. Name Symbol Protons Neutrons Electrons Atomic Number Mass Number Table $1$: Atoms of the First Six Elements Hydrogen $\ce{H}$ 1 0 1 1 1 Helium $\ce{He}$ 2 2 2 2 4 Lithium $\ce{Li}$ 3 4 3 3 7 Beryllium $\ce{Be}$ 4 5 4 4 9 Boron $\ce{B}$ 5 6 5 5 11 Carbon $\ce{C}$ 6 6 6 6 12 Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since $2 + 2 = 4$, we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, leaving it with a mass number of 7. Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction. $\text{Number of neutrons} = \text{mass number} - \text{atomic number}\nonumber$ Example $1$ Atoms of the element chromium $\left( \ce{Cr} \right)$ have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? Solution To determine this, you subtract the atomic number from the mass number, as shown above: $52 - 24 = 28 \: \text{neutrons in a chromium atom}\nonumber$ The composition of any atom can be illustrated with a shorthand notation using the atomic number and the mass number. Both are written before the chemical symbol, with the mass number written as a superscript and the atomic number written as a subscript. The chromium atom discussed above would be written as: $\ce{^{52}_{24}Cr}\nonumber$ Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. The above atom would be written as chromium-52. Summary • The mass number is defined as the total number of protons and neutrons in an atom. • The mass number = number of neutrons + atomic number. Review 1. Who first determined atomic weights for elements? 2. What were the original atomic weights based on? 3. Why were calculations based on numbers of protons not valid for determining atomic weights? 4. A tin atom has an atomic number of 50 and a mass number of 118. How many neutrons are present in this atom? 5. What is the mass number of a cobalt atom that has 27 protons and 30 neutrons?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.17%3A_Mass_Number.txt
Are all the members of the football team shown above identical? They are on the same team and are all known by the same team name, but there are individual differences among the players. We do not expect the kicker to be as big as the quarterback. The tight end is very likely to weigh less than the defensive tackle on the other side of the ball. They play as a unit, but they have different weights and heights. What are Isotopes? The history of the atom is full of some of these differences. Although John Dalton stated in his atomic theory of 1804 that all atoms of an element are identical, the discovery of the neutron began to show that this assumption was not correct. The study of radioactive materials (elements that spontaneously give off particles to form new elements) by Frederick Soddy (1877-1956) gave important clues about the internal structure of atoms. His work showed that some substances with different radioactive properties and different atomic masses were in fact the same element. He coined the term isotope from the Greek roots isos (íσος “equal”) and topos (τóπος “place”). He described isotopes as, “Put colloquially, their atoms have identical outsides but different insides.” Soddy won the Nobel Prize in Chemistry in 1921 for his work. As stated earlier, not all atoms of a given element are identical. Specifically, the number of neutrons can be variable for many elements. As an example, naturally occurring carbon exists in three forms. Each carbon atom has the same number of protons (6), which is its atomic number. Each carbon atom also contains six electrons in order to maintain electrical neutrality. However the number of neutrons varies as six, seven, or eight. Isotopes are atoms that have the same number atomic number, but different mass numbers due to a change in the number of neutrons. The three isotopes of carbon can be referred to as carbon-12 ($^{12} _6C$), carbon-13 ($^{13} _6C$), and carbon-14 ($^{14} _6C$) refers to the nucleus of a given isotope of an element. A carbon atom is one of three different nuclides. Most elements naturally consist of mixtures of isotopes. Carbon has three natural isotopes, while some heavier elements can have many more. Tin has ten stable isotopes, the most of any element. While the presence of isotopes affects the mass of an atom, it does not affect its chemical reactivity. Chemical behavior is governed by the number of electrons and the number of protons. Carbon-13 behaves chemically in exactly the same way as the more plentiful carbon-12. Summary • Isotopes are atoms that have the same atomic number, but different mass numbers due to a change in the number of neutrons. • The term nuclide refers to the nucleus of a given isotope of an element. • The atomic mass of an atom equals the sum of the protons and the neutrons. Review 1. What are isotopes? 2. Why do different isotopes of an element generally have the same physical and chemical properties? 3. How would the nucleus of the hydrogen-1 and hydrogen-2 differ? 4. Relate the concepts of isotope and mass number. 5. All oxygen atoms have eight protons, and most have eight neutrons as well. What is the mass number of an oxygen isotope that has nine neutrons? What is the name of this isotope? 6. An isotope of yttrium has 39 protons and 59 neutrons. What is the mass number of that isotope? 7. An isotope with a mass number of 193 has 116 neutrons. What is the atomic number of this isotope? 8. An isotope of barium (atomic number 56) has an mass of 138. How many neutrons are in the nucleus of this isotope? 9. Relate the concepts of isotope and mass number.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.18%3A_Isotopes.txt
The current system of measurement in the United States is a hodge-podge of different units, many of which are hard to interconvert. We at least have standardized units these days, unlike centuries past. At one time, measurements of length were defined as the distance from the end of one appendage to another. For example, the yard was defined as the distance from the king's nose to the tip of his thumb, when his arm was stretched out. Standardized measurements make it possible for people everywhere to get the same amount of something. Note the red and white labels on the gas pump pictured below. These labels certify that the gas pump has been checked, and is pumping an accurate gallon of gas. Standard measurements in science are very important so that we can compare experimental data from one lab to another, and make sure that we are all talking about the same thing. Atomic Mass Masses of individual atoms are very, very small. Using a modern device called a mass spectrometer, it is possible to measure such miniscule masses. An atom of oxygen-16, for example, has a mass of $2.66 \times 10^{-23} \: \text{g}$. While comparisons of masses measured in grams would have some usefulness, it is far more practical to have a system that will allow us to more easily compare relative atomic masses. Scientists decided on using the carbon-12 nuclide as the reference standard by which all other masses would be compared. By definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units $\left( \text{amu} \right)$. An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12. The mass of any isotope of any element is expressed in relation to the carbon-12 standard. For example, one atom of helium-4 has a mass of $4.0026 \: \text{amu}$. An atom of sulfur-32 has a mass of $31.972 \: \text{amu}$. The carbon-12 atom has six protons and six neutrons in its nucleus for a mass number of 12. Since the nucleus accounts for nearly all of the mass of the atom, a single proton or single neutron has a mass of approximately $1 \: \text{amu}$. However, as seen by the helium and sulfur examples, the masses of individual atoms are not whole numbers. This is because an atom's mass is affected very slightly by the interactions of the various particles within the nucleus, and the small mass of the electron is taken into account. Summary • Carbon-12 is the reference for all atomic mass calculations. • An atomic mass unit is defined as a mass equal to one twelfth the mass of an atom of carbon-12. • The mass of an atom is affected by the interactions of particles within the nucleus. Review 1. What instrument is used to measure the mass of atoms? 2. How much does a single oxygen-16 atom weigh in grams? 3. What is the reference standard for atomic mass units? 4. How is an atomic mass unit defined? 5. Why are the numbers for atomic mass of individual atoms not whole numbers?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.19%3A_Atomic_Mass_Unit.txt
Have you ever tried to move a boulder? You have a pile of rocks to move and need to decide what equipment you want to rent to move them. If the rocks are fairly small, you can get a shovel to pick them up. Larger rocks could be moved by hand, but big boulders will need some sort of mechanical scoop. The amount of each kind of rock will also determine how much time you will need to get the job done. Knowing the relative amounts of large, medium, and small rocks can be very useful in deciding how to approach the job. Percent Natural Abundance Most elements occur naturally as a mixture of two or more isotopes. The table below shows the natural isotopes of several elements, along with the percent natural abundance of each. Element Isotope (Symbol) Percent Natural Abundance Atomic Mass $\left( \text{amu} \right)$ Average Atomic Mass $\left( \text{amu} \right)$ Table $1$: Atomic Masses and Percents of Abundance of Some Natural Isotopes Hydrogen $\ce{_1^1H}$ 99.985 1.0078 1.0080 $\ce{_1^2H}$ 0.015 2.0141 $\ce{_1^3H}$ negligible 3.0160 Carbon $\ce{_6^{12}C}$ 98.89 12.000 12.011 $\ce{_6^{13}C}$ 1.11 13.003 $\ce{_6^{14}C}$ trace 14.003 Oxygen $\ce{_8^{16}O}$ 99.759 15.995 15.999 $\ce{_8^{17}O}$ 0.037 16.995 $\ce{_8^{18}O}$ 0.204 17.999 Chlorine $\ce{_{17}^{35}Cl}$ 75.77 34.969 35.453 $\ce{_{17}^{37}Cl}$ 24.23 36.966 Copper $\ce{_{29}^{63}Cu}$ 69.17 62.930 63.546 $\ce{_{29}^{65}Cu}$ 30.83 64.928 For some elements, one particular isotope predominates greatly over the other isotopes. Naturally occurring hydrogen is nearly all hydrogen-1 and naturally occurring oxygen is nearly all oxygen-16. For many other elements, however, more than one isotope may exist in more substantial quantities. Chlorine (atomic number 17) is a yellowish-green toxic gas. About three quarters of all chlorine atoms have 18 neutrons, giving those atoms a mass number of 35. About one quarter of all chlorine atoms have 20 neutrons, giving those atoms a mass number of 37. Were you to simply calculate the arithmetic average of the precise atomic masses, you would get 36. $\frac{\left( 34.969 + 36.966 \right)}{2} = 35.968 \: \text{amu}\nonumber$ Clearly the actual average atomic mass from the last column of the table is significantly lower. Why? We need to take into account the percent natural abundance of each isotope, in order to calculate the weighted average. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The sample problem below demonstrates how to calculate the atomic mass of chlorine. Example $1$ Use the atomic masses of each of the two isotopes of chlorine along with their respective percent abundances to calculate the average atomic mass of chlorine. Known • Chlorine-35: atomic mass $= 34.969 \: \text{amu}$ and percent abundance $= 75.77\%$ • Chlorine-37: atomic mass $= 36.966 \: \text{amu}$ and percent abundance $= 24.23\%$ Unknown • Average atomic mass of chlorine Change each percent abundance into decimal form by dividing by 100. Multiply this value by the atomic mass of that isotope. Add together for each isotope to get the average atomic mass. Step 2: Calculate. $\begin{array}{ll} \text{chlorine-35} & 0.7577 \times 34.969 = 26.50 \: \text{amu} \ \text{chlorine-37} & 0.2423 \times 36.966 = 8.957 \: \text{amu} \ \text{average atomic mass} & 26.50 + 8.957 = 35.46 \: \text{amu} \end{array}\nonumber$ Note: Applying significant figure rules results in the $35.45 \: \text{amu}$ result without excessive rounding error. In one step: $\left( 0.7577 \times 34.969 \right) + \left(0.2423 \times 36.966 \right) = 35.46 \: \text{amu}\nonumber$ Step 3: Think about your result. The calculated average atomic mass is closer to 35 than to 37 because a greater percentage of naturally occurring chlorine atoms have the mass number of 35. It agrees with the value from the table above. Summary • The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. • Calculations of atomic mass use the percent abundance of each isotope. Review 1. Define atomic mass. 2. What information do you need to calculate atomic mass for an element? 3. Calculate the atomic mass for carbon using the data provided in the table below. Isotope Atomic Mass Percent Abundance carbon-12 12.000000 98.90 carbon-13 13.003355 1.100	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/04%3A_Atomic_Structure/4.20%3A_Calculating_Average_Atomic_Mass.txt
• 5.1: Electromagnetic Spectrum • 5.2: Wavelength and Frequency Calculations • 5.3: Quantization of Energy • 5.4: Photoelectric Effect The photoelectric effect is a phenomenon that occurs when light shined onto a metal surface causes the ejection of electrons from that metal. It was observed that only certain frequencies of light are able to cause the ejection of electrons. If the frequency of the incident light is too low (red light, for example), then no electrons were ejected even if the intensity of the light was very high or it was shone onto the surface for a long time. • 5.5: Atomic Emission Spectra • 5.6: Bohr's Atomic Model • 5.7: Spectral Lines of Atomic Hydrogen • 5.8: de Broglie Wave Equation • 5.9: Quantum Mechanics Quantum mechanics is the study of the motion of objects that are atomic or subatomic in size and thus demonstrate wave-particle duality. In classical mechanics, the size and mass of the objects involved effectively obscures any quantum effects so that such objects appear to gain or lose energies in any amounts. Particles whose motion is described by quantum mechanics gain or lose energy in the small pieces called quanta. • 5.10: Heisenberg Uncertainty Principle The Heisenberg Uncertainty Principle explains why we cannot simultaneously determine both the precise velocity and position of a particle. This principle is only applicable at the atomic level. • 5.11: Quantum Mechanical Atomic Model The quantum mechanical model of the atom comes from the solution to Schrödinger's equation. Quantization of electron energies is a requirement in order to solve the equation. Solutions to the Schrödinger wave equation, called wave functions, give only the probability of finding an electron at a given point around the nucleus. Electrons do not travel around the nucleus in simple circular orbits. • 5.12: Energy Level • 5.13: Orbitals We can apply our knowledge of quantum numbers to describe the arrangement of electrons for a given atom. We do this with something called electron configurations. They are effectively a map of the electrons for a given atom. We look at the four quantum numbers for a given electron and then assign that electron to a specific orbital in the next Module. • 5.14: Quantum Numbers We use a series of specific numbers, called quantum numbers, to describe the location of an electron in an associated atom. Quantum numbers specify the properties of the atomic orbitals and the electrons in those orbitals. An electron in an atom or ion has four quantum numbers to describe its state. Think of them as important variables in an equation which describes the three-dimensional position of electrons in a given atom. • 5.15: Aufbau Principle to create ground state electron configurations for any element, it is necessary to know the way in which the atomic sublevels are organized in order of increasing energy. The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest. The Aufbau (German: "building up, construction") principle is sometimes referred to as the "building up" principle. • 5.16: Pauli Exclusion Principle The Pauli exclusion principle, which states that no two electrons in an atom can have the same set of four quantum numbers. The energy of the electron is specified by the principal, angular momentum, and magnetic quantum numbers. If those three numbers are identical for two electrons, the spin numbers must be different in order for the two electrons to be differentiated from one another. • 5.17: Hund's Rule and Orbital Filling Diagrams Hund's rule states that orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron and that each of the single electrons must have the same spin. An orbital filling diagram is the more visual way to represent the arrangement of all the electrons in a particular atom. In an orbital filling diagram, the individual orbitals are shown as circles (or squares) and orbitals within a sublevel are drawn next to each other horizontally. • 5.18: Electron Configurations Electron configuration notation eliminates the boxes and arrows of orbital filling diagrams. Each occupied sublevel designation is written followed by a superscript that is the number of electrons in that sublevel. • 5.19: Valence Electrons Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the 1s sublevel are called inner-shell electrons and are not involved directly in the element's reactivity or in the formation of compounds. • 5.20: Noble Gas Configuration A noble gas configuration of an atom consists of the elemental symbol of the last noble gas prior to that atom, followed by the configuration of the remaining electrons. 05: Electrons in Atoms What are waves? Waves can come in many sizes. Here we see a large wave crashing on the beach. Other waves can be very small and regular. We normally think of waves as being made of water, but there are forms of energy that take on the characteristics of waves. The idea of a wave has played a major role in our understanding of how the atom is put together and why it behaves the way it does. Properties of Light The nuclear atomic model proposed by Rutherford was a great improvement over previous models, but was still not complete. It did not fully explain the location and behavior of the electrons in the vast space outside of the nucleus. In fact, it was well known that oppositely charged particles attract one another. Rutherford's model did not explain why the electrons don't simply move toward, and eventually collide with, the nucleus. Experiments in the early twentieth century began to focus on the absorption and emission of light by matter. These studies showed how certain phenomena associated with light reveal insight into the nature of matter, energy, and atomic structure. Wave Nature of Light In order to begin to understand the nature of the electron, we first need to look at the properties of light. Prior to 1900, scientists thought light behaved solely as a wave. As we will see later, this began to change as new experiments demonstrated that light also has some of the characteristics of a particle. First, we will examine the wavelike properties of light. Visible light is one type of electromagnetic radiation, which is a form of energy that exhibits wavelike behavior as it moves through space. Other types of electromagnetic radiation include gamma rays, x-rays, ultraviolet light, infrared light, microwaves, and radio waves. The figure below shows the electromagnetic spectrum, which is all forms of electromagnetic radiation. Notice that visible light makes up only a very, very small portion of the entire electromagnetic spectrum. All electromagnetic radiation moves through a vacuum at a constant speed of $2.998 \times 10^8 \: \text{m/s}$. While the presence of air molecules slows the speed of light by a very small amount, we will use the value of $3.00 \times 10^8 \: \text{m/s}$ as the speed of light in air. The figure above shows how the electromagnetic spectrum displays a wide variation in wavelength and frequency. Radio waves have wavelengths of as long as hundreds of meters, while the wavelength of gamma rays are on the order of $10^{-12} \: \text{m}$. The corresponding frequencies range from $10^6$ to $10^{21} \: \text{Hz}$. Visible light can be split into colors with the use of a prism (see below), yielding the visible spectrum of light. Red light has the longest wavelength and lowest frequency, while violet light has the shortest wavelength and highest frequency. Visible light wavelength ranges from about $400$ to $700 \: \text{nm}$ with frequencies in the range of $10^{14} \: \text{Hz}$. Simulation If we could see other wavelengths of energy, would they appear as colors? Use this simulation to explore the electromagnetic spectrum. Summary • Electromagnetic radiation is a form of energy. • Visible light has wavelengths from $400$-$700 \: \text{nm}$. • The speed of light in air is $3.00 \times 10^8 \: \text{m/s}$. Review 1. What did Rutherford’s nuclear atomic model not explain? 2. Prior to 1900, what did scientists believe about the nature of light? 3. What is visible light? 4. What is the range of wavelengths for visible light?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.01%3A_Electromagnetic_Spectrum.txt
Do you enjoy going to the beach? During the summer, almost everyone enjoys going to the beach. Beach-goers can swim, have picnics, and work on their tans. But if a beach-goer gets too much sun, they can burn. A particular set of solar wavelengths are especially harmful to the skin. This portion of the solar spectrum is known as UV B, with wavelengths of $280$-$320 \: \text{nm}$. Sunscreens are effective in protecting skin against both the immediate skin damage and the long-term possibility of skin cancer. Waves Waves are characterized by their repetitive motion. Imagine a toy boat riding the waves in a wave pool. As the water wave passes under the boat, it moves up and down in a regular and repetitive fashion. While the wave travels horizontally, the boat only travels vertically up and down. The figure below shows two examples of waves. A wave cycle consists of one complete wave—starting at the zero point, going up to a wave crest, going back down to a wave trough, and back to the zero point again. The wavelength of a wave is the distance between any two corresponding points on adjacent waves. It is easiest to visualize the wavelength of a wave as the distance from one wave crest to the next. In an equation, wavelength is represented by the Greek letter lambda $\left( \lambda \right)$. Depending on the type of wave, wavelength can be measured in meters, centimeters, or nanometers $\left( 1 \: \text{m} = 10^9 \: \text{nm} \right)$. The frequency, represented by the Greek letter nu $\left( \nu \right)$, is the number of waves that pass a certain point in a specified amount of time. Typically, frequency is measured in units of cycles per second or waves per second. One wave per second is also called a Hertz $\left( \text{Hz} \right)$ and in SI units is a reciprocal second $\left( \text{s}^{-1} \right)$. Figure B above shows an important relationship between the wavelength and frequency of a wave. The top wave clearly has a shorter wavelength than the second wave. However, if you picture yourself at a stationary point watching these waves pass by, more waves of the first kind would pass by in a given amount of time. Thus the frequency of the first wave is greater than that of the second wave. Wavelength and frequency are therefore inversely related. As the wavelength of a wave increases, its frequency decreases. The equation that relates the two is: $c = \lambda \nu\nonumber$ The variable $c$ is the speed of light. For the relationship to hold mathematically, if the speed of light is used in $\text{m/s}$, the wavelength must be in meters and the frequency in Hertz. Example $1$ The color orange within the visible light spectrum has a wavelength of about $620 \: \text{nm}$. What is the frequency of orange light? Known • Wavelength $\left( \lambda \right) = 620 \: \text{nm}$ • Speed of light $\left( c \right) = 3.00 \times 10^8 \: \text{m/s}$ • Conversion factor $1 \: \text{m} = 10^9 \: \text{nm}$ Unknown • Frequency Convert the wavelength to $\text{m}$, then apply the equation $c = \lambda \nu$ and solve for frequency. Dividing both sides of the equation by $\lambda$ yields: $\nu = \frac{c}{\lambda}\nonumber$ Step 2: Calculate. $620 \: \text{nm} \times \left( \frac{1 \: \text{m}}{10^9 \: \text{nm}} \right) = 6.20 \times 10^{-7} \: \text{m}\nonumber$ $\nu = \frac{c}{\lambda} = \frac{3.0 \times 10^8 \: \text{m/s}}{6.20 \times 10^{-7}} = 4.8 \times 10^{14} \: \text{Hz}\nonumber$ Step 3: Think about your result. The value for the frequency falls within the range for visible light. Summary • All waves can be defined in terms of their frequency and intensity. • $c = \lambda \nu$ expresses the relationship between wavelength and frequency. Review 1. Define wavelength. 2. Define frequency. 3. What is the relationship between wavelength and frequency?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.02%3A_Wavelength_and_Frequency_Calculations.txt
Every so often, you may hear a commercial or a news story with the words "quantum leap" in it. The quantum leap is supposed to be a major breakthrough—a big change, something extraordinarily large. The reality is far different. Instead of a big extravagant change, the "quantum" that scientists know about is a very small difference in the location of an electron around a nucleus—hardly an enormous shift at all. Quantization of Energy German physicist Max Planck (1858-1947) studied the emission of light by hot objects. You have likely seen a heated metal object glow an orange-red color (see below). Classical physics, which explains the behavior of large, everyday objects, predicted that a hot object would emit electromagnetic energy in a continuous fashion. In other words, every wavelength of light could possibly be emitted. Instead, what Planck found by analyzing the spectra was that the energy of the hot body could only be lost in small discrete units. A quantum is the minimum quantity of energy that can either be lost or gained by an atom. An analogy is that a brick wall can only undergo a change in height by units of one or more bricks, and not by any possible height. Planck showed that the amount of radiant energy absorbed or emitted by an object is directly proportional to the frequency of the radiation. $E = h \nu\nonumber$ In the equation, $E$ is the energy, in joules, of a quantum of radiation, $\nu$ is the frequency, and $h$ is a fundamental constant called Planck's constant. The value of Planck's constant is $h = 6.626 \times 10^{-34} \: \text{J} \cdot \text{s}$. The energy of any system must increase or decrease in units of $h \times \nu$. A small energy change results in the emission or absorption of low-frequency radiation, while a large energy change results in the emission or absorption of high-frequency radiation. Example $1$ What is the energy of a photon of green light with a frequency of $5.75 \times 10^{14} \: \text{Hz}$? Known • Frequency $\left( \nu \right) = 5.75 \times 10^{14} \: \text{Hz}$ • Planck's constant $\left( h \right) = 6.626 \times 10^{-34} \: \text{J} \cdot \text{s}$ Unknown • energy ($E$) Apply the equation $E = h \nu$ to solve for the energy. Step 2: Calculate. $E = \left( 6.626 \times 10^{-34} \: \text{J} \cdot \text{s} \right) \times \left( 5.75 \times 10^{14} \: \text{Hz} \right) = 3.81 \times 10^{-19} \: \text{J}\nonumber$ Step 3: Think about your result. While the resulting energy may seem very small, this is for only one photon of light. Visible quantities of light consist of huge quantities of photons. Recall that a hertz is equal to a reciprocal second, so the units agree in the equation. Summary • A quantum is the minimum quantity of energy that can be lost or gained by an atom. • The value of Planck's constant is $h = 6.626 \times 10^{-34} \: \text{J} \cdot \text{s}$. Review 1. What did classical physics predict about hot objects? 2. What did Planck find? 3. What is a quantum? 5.04: Photoelectric Effect Are you a fan of science fiction? In 1950s science fiction stories, one space travel theme was the use of solar sails for propulsion. The idea was that the photon pressure from the sun would push the sail (like wind sails) and move the spacecraft. What once was science fiction is now reality, as solar sails are being developed and tested for modern space travel. Photoelectric Effect and the Particle Nature of Light In 1905, Albert Einstein (1879-1955) proposed that light be described as quanta of energy that behave as particles. A photon is a particle of electromagnetic radiation that has zero mass and carries a quantum of energy. The energy of photons of light is quantized according to the $E = h \nu$ equation. For many years light had been described using only wave concepts, and scientists trained in classical physics found this wave-particle duality of light to be a difficult idea to accept. A key concept that was explained by Einstein using light's particle nature was called the photoelectric effect. The photoelectric effect is a phenomenon that occurs when light shone onto a metal surface causes the ejection of electrons from that metal. It was observed that only certain frequencies of light are able to cause the ejection of electrons. If the frequency of the incident light was too low (red light, for example), then no electrons were ejected, even if the intensity of the light was very high or it was shone onto the surface for a long time. If the frequency of the light was higher (green light, for example), then electrons were able to be ejected from the metal surface even if the intensity was very low or it was shone for only a short time. This minimum frequency needed to cause electron ejection is referred to as the threshold frequency. Classical physics was unable to explain the photoelectric effect. If classical physics was applied to this situation, the electron in the metal could eventually collect enough energy to be ejected from the surface even if the incoming light was of low frequency. Einstein used the particle theory of light to explain the photoelectric effect as shown in the figure below. Consider the $E = h \nu$ equation. The $E$ is the minimum energy that is required in order for the metal's electron to be ejected. If the incoming light's frequency, $\nu$, is below the threshold frequency, there will never be enough energy to cause electrons to be ejected. If the frequency is equal to or higher than the threshold frequency, electrons will be ejected. As the frequency increases beyond the threshold, the ejected electrons simply move faster. An increase in the intensity of incoming light that is above the threshold frequency causes the number of electrons that are ejected to increase, but they do not travel any faster. The photoelectric effect is applied in devices called photoelectric cells, which are commonly found in everyday items (such as a calculator) that use the energy of light to generate electricity. Simulation Launch the PLIX interactive below to determine which type of colored light has enough energy (eV) to eject an electron from the metal plate: Summary • Light has properties of both a wave and a particle. • The photoelectric effect is produced by light striking a metal and dislodging electrons from the surface of the metal. Review 1. What are the properties of a photon? 2. What does the photoelectric effect show about the properties of light? 3. How does the frequency of light affect the release of photons?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.03%3A_Quantization_of_Energy.txt
How much energy does it take to shoot an arrow? Archery as a sport or a means of defense has existed for centuries. At rest, there is no tension on the bowstring and no force on the arrow. When the string and arrow are pulled back, we now have a situation where kinetic energy (pulling of the string) has been converted to potential energy (the tension on the string). The archer releases the arrow and the potential energy is translated into kinetic energy as the arrow moves. It turns out that electrons behave the same way when energy is put into the system or released from the system. Atomic Emission Spectra The electrons in an atom tend to be arranged in such a way that the energy of the atom is as low as possible. The ground state of an atom is the lowest energy state of the atom. When those atoms are given energy, the electrons absorb the energy and move to a higher energy level. These energy levels of the electrons in atoms are quantized, meaning again that the electron must move from one energy level to another in discrete steps, rather than continuously. An excited state of an atom is a state where its potential energy is higher than the ground state. An atom in the excited state is not stable. When it returns back to the ground state, it releases the energy that it had previously gained in the form of electromagnetic radiation. So how do atoms gain energy in the first place? One way is to pass an electric current through an enclosed sample of a gas at low pressure. Since the electron energy levels are unique for each element, every gas discharge tube will glow with a distinctive color, depending on the identity of the gas (see below). "Neon" signs are familiar examples of gas discharge tubes. However, only signs that glow with the red-orange color seen in the figure are actually filled with neon. Signs of other colors contain different gases or mixtures of gases. Scientists studied the distinctive pink color of the gas discharge created by hydrogen gas. When a narrow beam of this light was viewed through a prism, the light was separated into four lines of very specific wavelengths (and frequencies since $\lambda$ and $\nu$ are inversely related). An atomic emission spectrum is the pattern of lines formed when light passes through a prism to separate it into the different frequencies of light it contains. The figure below shows the atomic emission spectrum of hydrogen. Classical theory was unable to explain the existence of atomic emission spectra, also known as line-emission spectra. According to classical physics, a ground state atom would be able to absorb any amount of energy rather than only discrete amounts. Likewise, when the atoms relaxed back to a lower energy state, any amount of energy could be released. This would result in what is known as a continuous spectrum, where all wavelengths and frequencies are represented. White light viewed through a prism and a rainbow are examples of continuous spectra. Atomic emission spectra were more proof of the quantized nature of light and led to a new model of the atom based on quantum theory. Simulation Look up at the sky at night and you'll see stars. How do we know what they are made of? Check out this simulation explore atomic colors and the ingredients that make up our universe. Summary • Atomic emission spectra are produced when excited electrons return to the ground state. • The emitted light of electrons corresponds to energies of the specific electrons. Review 1. What is the ground state of an atom? 2. What is an excited state? 3. Why do we see emission lines when electrons return to the ground state?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.05%3A_Atomic_Emission_Spectra.txt
How does this worker's energy change as he climbs up and down the ladder? Climbing a ladder takes energy. At every step, you push yourself up against gravity, and accumulate potential energy. Coming back down releases that potential energy as you descend step by step. If you are not careful, you can release that potential energy all at once by falling off the ladder (never a good thing). In addition, you take the climb or descent in steps. There is no "in-between" position on the ladder—your foot either hits a rung, or it hits empty space, and you are in trouble until you find a rung to stand on. Bohr's Atomic Model Following the discoveries of hydrogen emission spectra and the photoelectric effect, the Danish physicist Niels Bohr (1885-1962) proposed a new model of the atom in 1915. Bohr proposed that electrons do not radiate energy as they orbit the nucleus, but exist in states of constant energy that he called stationary states. This means that the electrons orbit at fixed distances from the nucleus (see below). Bohr's work was primarily based on the emission spectra of hydrogen. This is also referred to as the planetary model of the atom. It explained the inner workings of the hydrogen atom. Bohr was awarded the Nobel Prize in physics in 1922 for his work. Bohr explained that electrons can be moved into different orbits with the addition of energy. When the energy is removed, the electrons return back to their ground state, emitting a corresponding amount of energy—a quantum of light, or photon. This was the basis for what later became known as quantum theory. This is a theory based on the principle that matter and energy have the properties of both particles and waves. It accounts for a wide range of physical phenomena, including the existence of discrete packets of energy and matter, the uncertainty principle, and the exclusion principle. According to the Bohr model, often referred to as a planetary model, the electrons encircle the nucleus of the atom in specific allowable paths called orbits. When the electron is in one of these orbits, its energy is fixed. The ground state of the hydrogen atom, where its energy is lowest, is when the electron is in the orbit that is closest to the nucleus. The orbits that are further from the nucleus are all of successively greater energy. The electron is not allowed to occupy any of the spaces in between the orbits. An everyday analogy to the Bohr model is the rungs of a ladder. As you move up or down a ladder, you can only occupy specific rungs and cannot be in the spaces in between rungs. Moving up the ladder increases your potential energy, while moving down the ladder decreases your energy. Bohr's work had a strong influence on our modern understanding of the inner workings of the atom. However, his model worked well as an explanation for the emissions of the hydrogen atom, but was seriously limited when applied to other atoms. Shortly after Bohr published his planetary model of the atom, several new discoveries were made, which resulted in, yet again, a revised view of the atom. Summary • The Bohr model postulates that electrons orbit the nucleus at fixed energy levels. • Orbits further from the nucleus exist at higher energy levels. • When electrons return to a lower energy level, they emit energy in the form of light. Review 1. When did Bohr propose his model of the atom? 2. What is a stationary state? 3. What is the ground state? 4. Can the electron occupy any space between the orbits?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.06%3A_Bohr%27s_Atomic_Model.txt
Can you fix a car engine? While less common in this day and age, there was a time when many people could work on their own cars if there was a problem. Today, engines are computerized and require specialized training and tools in order to be fixed. When people did their own repairs, it was sometimes a trial and error process. Maybe the spark plugs needed to be replaced. No, that didn’t fix the problem completely, but it was a start in the right direction. Science operates the same way. A theory that is developed may work for a while, but then there are data that the theory cannot explain. This means that it's time for a newer and more inclusive theory. Spectral Lines of Hydrogen Bohr's model explains the spectral lines of the hydrogen atomic emission spectrum. While the electron of the atom remains in the ground state, its energy is unchanged. When the atom absorbs one or more quanta of energy, the electron moves from the ground state orbit to an excited state orbit that is further away. Energy levels are designated with the variable $n$. The ground state is $n=1$, the first excited state is $n=2$, and so on. The energy that is gained by the atom is equal to the difference in energy between the two energy levels. When the atom relaxes back to a lower energy state, it releases energy that is again equal to the difference in energy of the two orbits (see below). The change in energy, $\Delta E$, then translates to light of a particular frequency being emitted according to the equation $E = h \nu$. Recall that the atomic emission spectrum of hydrogen had spectral lines consisting of four different frequencies. This is explained in the Bohr model by the realization that the electron orbits are not equally spaced. As the energy increases further and further from the nucleus, the spacing between the levels gets smaller and smaller. Based on the wavelengths of the spectral lines, Bohr was able to calculate the energies that the hydrogen electron would have in each of its allowed energy levels. He then mathematically showed which energy level transitions correspond to the spectral lines in the atomic emission spectrum (see below). He found that the four visible spectral lines corresponded to transitions from higher energy levels down to the second energy level $\left( n=2 \right)$. This is called the Balmer series. Transitions ending in the ground state $\left( n=1 \right)$ are called the Lyman series, but the energies released are so large that the spectral lines are all in the ultraviolet region of the spectrum. The transitions called the Paschen series and the Brackett series both result in spectral lines in the infrared region, because the energies are too small. Bohr's model was a tremendous success in explaining the spectrum of the hydrogen atom. Unfortunately, when the mathematics of the model were applied to atoms with more than one electron, it was not able to correctly predict the frequencies of the spectral lines. While Bohr's model represented a great advancement in the atomic model and the concept of electron transitions between energy levels is valid, improvements were needed in order to fully understand all atoms and their chemical behavior. Summary • Emission lines for hydrogen correspond to energy changes related to electron transitions. • The Bohr model works only for the hydrogen atom. Review 1. What happens when a hydrogen atom absorbs one or more quanta of energy? 2. How do we detect the change in energy? 3. What electron transitions are presented by the lines of the Paschen series? 4. Does the Bohr model work for atoms other than hydrogen?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.07%3A_Spectral_Lines_of_Atomic_Hydrogen.txt
Bohr's model of the atom was valuable in demonstrating how electrons were capable of absorbing and releasing energy, and how atomic emission spectra were created. However, the model did not really explain why electrons should exist only in fixed circular orbits, rather than being able to exist in a limitless number of orbits with different energies. In order to explain why atomic energy states are quantized, scientists had to rethink their views of the nature of the electron and its movement. de Broglie Wave Equation Planck's investigation of the emission spectra of hot objects and the subsequent studies into the photoelectric effect had proven that light was capable of behaving both as a wave and as a particle. It seemed reasonable to wonder if electrons could also have a dual wave-particle nature. In 1924, French scientist Louis de Broglie (1892-1987) derived an equation that described the wave nature of any particle. Particularly, the wavelength $\left( \lambda \right)$ of any moving object is given by: $\lambda = \frac{h}{mv}\nonumber$ In this equation, $h$ is Planck's constant, $m$ is the mass of the particle in $\text{kg}$, and $v$ is the velocity of the particle in $\text{m/s}$. The problem below shows how to calculate the wavelength of the electron. Example $1$ An electron of mass $9.11 \times 10^{-31} \: \text{kg}$ moves at nearly the speed of light. Using a velocity of $3.00 \times 10^8 \: \text{m/s}$, calculate the wavelength of the electron. Solution Step 1: List the known quantities and plan the problem. Known • Mass $\left( m \right) = 9.11 \times 10^{-31} \: \text{kg}$ • Planck's constant $\left( h \right) = 6.626 \times 10^{-34} \: \text{J} \cdot \text{s}$ • Velocity $\left( v \right) = 3.00 \times 10^8 \: \text{m/s}$ Unknown • Wavelength ($\lambda$) Apply the de Broglie wave equation $\lambda = \frac{h}{mv}$ to solve for the wavelength of the moving electron. Step 2: Calculate. $\lambda = \frac{h}{mv} = \frac{6.626 \times 10^{-34} \: \text{J} \cdot \text{s}}{\left( 9.11 \times 10^{-31} \: \text{kg} \right) \times \left( 3.00 \times 10^8 \: \text{m/s} \right)} = 2.42 \times 10^{-12} \: \text{m}\nonumber$ Step 3: Think about your result. This very small wavelength is about 1/20 of the diameter of a hydrogen atom. Looking at the equation, as the speed of the electron decreases, its wavelength increases. The wavelengths of everyday large objects with much greater masses should be very small. If we were to calculate the wavelength of a $0.145 \: \text{kg}$ baseball thrown at a speed of $40 \: \text{m/s}$, we would come up with an extremely short wavelength on the order of $10^{-34} \: \text{m}$. This wavelength is impossible to detect even with advanced scientific equipment. Indeed, while all objects move with wavelike motion, we never notice it because the wavelengths are far too short. On the other hand, particles with measurable wavelengths are all very small. However, the wave nature of the electron proved to be a key development in a new understanding of the nature of the electron. An electron that is confined to a particular space around the nucleus of an atom can only move around that atom in such a way that its electron wave "fits" the size of the atom correctly. This means that the frequencies of electron waves are quantized. Based on the $E = h \nu$ equation, the quantized frequencies mean that electrons can only exist in an atom at specific energies, as Bohr had previously theorized. Summary • The de Broglie wave equation allows the calculation of the wavelength of any moving object. • As the speed of the electron decreases, its wavelength increases. Review 1. What did the Bohr model not explain? 2. State the de Broglie wave equation. 3. What happens as the speed of the electron decreases?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.08%3A_de_Broglie_Wave_Equation.txt
How do you study something that seemingly makes no sense? We discuss electrons being in orbits, and it sounds like we can tell where an electron is at any moment. We can draw pictures of electrons in orbit, but the reality is that we just don't know exactly where they are. We will take a quick look at an area of science that even leaves scientists puzzled. When asked about quantum mechanics, Niels Bohr (who proposed the Bohr model of the atom) said, "Anyone who is not shocked by quantum theory has not understood it." Richard Feynman (one of the founders of modern quantum theory) stated, "I think I can safely say that nobody understands quantum theory." So, let's take a short trip into a land that challenges our everyday world... Quantum Mechanics The study of motion of large objects such as baseballs is called mechanics, or more specifically, classical mechanics. Because of the quantum nature of the electron and other tiny particles moving at high speeds, classical mechanics is inadequate to accurately describe their motion. Quantum mechanics is the study of the motion of objects that are atomic or subatomic in size and thus demonstrate wave-particle duality. In classical mechanics, the size and mass of the objects involved effectively obscures any quantum effects, so that such objects appear to gain or lose energies in any amounts. Particles whose motion is described by quantum mechanics gain or lose energy in the small pieces called quanta. One of the fundamental (and hardest to understand) principles of quantum mechanics is that the electron is both a particle and a wave. In the everyday macroscopic world of things we can see, something cannot be both. But this duality can exist in the quantum world of the submicroscopic on the atomic scale. At the heart of quantum mechanics is the idea that we cannot accurately specify the location of an electron. All we can say is that there is a probability that it exists within this certain volume of space. The scientist Erwin Schrödinger developed an equation that deals with these calculations, which we will not pursue at this time. Summary • Quantum mechanics involves the study of material at the atomic level where the particles motion is described by gaining or losing the discrete amounts called quanta. • In quantum mechanics, electrons exist simultaneously as both a particle and wave. • This field deals with probabilities since we cannot definitely locate a particle. Review 1. What does quantum mechanics help us understand? 2. How does quantum mechanics describe an electron? 3. According quantum mechanics, we cannot specify accurately the location of an electron. However what can we do to describe its location? 5.10: Heisenberg Uncertainty Principle Lasers have numerous applications outside the physics lab. These devices can be employed to measure distances accurately, and many commercial instruments are used in construction for laying out a building site. When the light beam strikes a solid object, it is reflected back, and the device determines how far away the object is. There is such a significant difference between the mass of the light beam (photons) and the mass of the object, that the beam does not disturb the object at all. A historically significant measurement of interest was the use of a laser to measure the distance from the Earth to the moon. The impact of the photons from the laser on the moon had absolutely no effect on the moon's orbit. Heisenberg Uncertainty Principle Another feature that is unique to quantum mechanics is the uncertainty principle. The Heisenberg Uncertainty Principle states that it is impossible to simultaneously determine both the position and the velocity of a particle. The detection of an electron, for example, would be made by way of its interaction with photons of light. Since photons and electrons have nearly the same energy, any attempt to locate an electron with a photon will knock the electron off course, resulting in uncertainty about where the electron is located (see below). We do not have to worry about the uncertainty principle with large everyday objects because of their mass. If you are looking for something with a flashlight, the photons coming from the flashlight are not going to cause the thing that you are looking for to move. However, this is not the case with atomic-sized particles, and has led scientists to a new understanding about how to envision the location of the electrons within atoms. Summary • The Heisenberg Uncertainty Principle explains why we cannot simultaneously determine both the precise velocity and position of a particle. • This principle is only applicable at the atomic level. Review 1. Why is the Heisenberg uncertainty principle true at the atomic level? 2. Is the principle valid at the macroscopic level?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.09%3A_Quantum_Mechanics.txt
Imagine that a news flash interrupts your favorite TV program... "There has been a hold-up at the First National Bank. The suspect fled in a car and is believed to be somewhere in the downtown district. Everyone is asked to be on the alert." The robber can be located only within a certain area - the police do not have an exact location, just a general idea as to the whereabouts of the thief. Quantum Mechanical Atomic Model In 1926, Austrian physicist Erwin Schrödinger (1887-1961) used the wave-particle duality of the electron to develop and solve a complex mathematical equation that accurately described the behavior of the electron in a hydrogen atom. The quantum mechanical model of the atom comes from the solution to Schrödinger's equation. Quantization of electron energies is a requirement in order to solve the equation. This is unlike the Bohr model, in which quantization was simply assumed with no mathematical basis. Recall that in the Bohr model, the exact path of the electron was restricted to very well-defined circular orbits around the nucleus. The quantum mechanical model is a radical departure from that. Solutions to the Schrödinger wave equation, called wave functions, give only the probability of finding an electron at a given point around the nucleus. Electrons do not travel around the nucleus in simple circular orbits. The location of the electrons in the quantum mechanical model of the atom is often referred to as an electron cloud. The electron cloud can be thought of in the following way: Imagine placing a square piece of paper on the floor with a dot in the circle representing the nucleus. Now take a marker and drop it onto the paper repeatedly, making small marks at each point the marker hits. If you drop the marker many, many times, the overall pattern of dots will be roughly circular. If you aim toward the center reasonably well, there will be more dots near the nucleus and progressively fewer dots as you move away from it. Each dot represents a location where the electron could be at any given moment. Because of the uncertainty principle, there is no way to know exactly where the electron is. An electron cloud has variable densities: a high density where the electron is most likely to be and a low density where the electron is least likely to be (see below). In order to specifically define the shape of the cloud, it is customary to refer to the area within which there is a $90%$ chance of finding the electron. This is called an orbital, the three-dimensional region of space that indicates where there is a high probability of finding an electron. Summary • The Schrödinger wave equation replaced Bohr's ideas about electron location with an uncertainty factor. • The location of the electron can only be given as a probability that the electron is somewhere in a certain area. Review 1. What does the quantum mechanical view of the atom require? 2. What is a wave function? 3. What does a high density electron cloud suggest?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.11%3A_Quantum_Mechanical_Atomic_Model.txt
Fireworks are a great way to celebrate happy events. Do you know what causes the brilliant, colored lights of fireworks? The lights are bursts of energy given off by atoms in the fireworks. What do you suppose causes these bursts of light? The answer has to do with energy levels of atoms. What Are Energy Levels? Energy levels (also called electron shells) are fixed distances from the nucleus of an atom where electrons may be found. Electrons are tiny, negatively charged particles in an atom that move around the positive nucleus at the center. Energy levels are a little like the steps of a staircase. You can stand on one step or another but not in between the steps. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model in the figure below shows the first four energy levels of an atom. Electrons in energy level I (also called energy level K) have the least amount of energy. As you go farther from the nucleus, electrons at higher levels have more energy, and their energy increases by a fixed, discrete amount. Electrons can jump from a lower to the next higher energy level if they absorb this amount of energy. Conversely, if electrons jump from a higher to a lower energy level, they give off energy, often in the form of light. This explains the fireworks pictured above. When the fireworks explode, electrons gain energy and jump to higher energy levels. When they jump back to their original energy levels, they release the energy as light. Different atoms have different arrangements of electrons, so they give off light of different colors. Q: In the atomic model Figure above, where would you find electrons that have the most energy? A: Electrons with the most energy would be found in energy level IV. Energy Levels and Orbitals The smallest atoms are hydrogen atoms. They have just one electron. That one electron is in the first energy level. Bigger atoms have more electrons. Electrons are always added to the lowest energy level first until it has the maximum number of electrons possible. Then electrons are added to the next higher energy level until that level is full, and so on. How many electrons can a given energy level hold? The maximum numbers of electrons possible for the first four energy levels are shown in the figure above. For example, energy level I can hold a maximum of two electrons, and energy level II can hold a maximum of eight electrons. The maximum number depends on the number of orbitals at a given energy level. An orbital is a volume of space within an atom where an electron is most likely to be found. As you can see by the images in the figure below, some orbitals are shaped like spheres (S orbitals) and some are shaped like dumbbells (P orbitals). There are other types of orbitals as well. Regardless of its shape, each orbital can hold a maximum of two electrons. Energy level I has just one orbital, so two electrons will fill this energy level. Energy level II has four orbitals, so it takes eight electrons to fill this energy level. Q: Energy level III can hold a maximum of 18 electrons. How many orbitals does this energy level have? A: At two electrons per orbital, this energy level must have nine orbitals. The Outermost Level Electrons in the outermost energy level of an atom have a special significance. These electrons are called valence electrons, and they determine many of the properties of an atom. An atom is most stable if its outermost energy level contains as many electrons as it can hold. For example, helium has two electrons, both in the first energy level. This energy level can hold only two electrons, so helium’s only energy level is full. This makes helium a very stable element. In other words, its atoms are unlikely to react with other atoms. Consider the elements fluorine and lithium, modeled in the figure below. Fluorine has seven of eight possible electrons in its outermost energy level, which is energy level II. It would be more stable if it had one more electron because this would fill its outermost energy level. Lithium, on the other hand, has just one of eight possible electrons in its outermost energy level (also energy level II). It would be more stable if it had one less electron because it would have a full outer energy level (now energy level I). Both fluorine and lithium are highly reactive elements because of their number of valence electrons. Fluorine will readily gain one electron and lithium will just as readily give up one electron to become more stable. In fact, lithium and fluorine will react together as shown in the figure below. When the two elements react, lithium transfers its one “extra” electron to fluorine. Q: A neon atom has ten electrons. How many electrons does it have in its outermost energy level? How stable do you think a neon atom is? A: A neon atom has two electrons in energy level I and its remaining eight electrons in energy level II, which can hold only eight electrons. This means that is outermost energy level is full. Therefore, a neon atom is very stable. Summary • Energy levels (also called electron shells) are fixed distances from the nucleus of an atom where electrons may be found. As you go farther from the nucleus, electrons at higher energy levels have more energy. • Electrons are always added to the lowest energy level first until it has the maximum number of electrons possible, and then electrons are added to the next higher energy level until that level is full, and so on. The maximum number of electrons at a given energy level depends on its number of orbitals. There are at most two electrons per orbital. • Electrons in the outermost energy level of an atom are called valence electrons. They determine many of the properties of an atom, including how reactive it is. Review 1. What are energy levels? 2. Relate energy levels to the amount of energy their electrons have. 3. What must happen for an electron to jump to a different energy level? 4. How many electrons can the fourth energy level have? How many orbitals are there at this energy level? 5. An atom of sodium has 11 electrons. Make a sketch of a sodium atom, showing how many electrons it has at each energy level. Infer how reactive sodium atoms are.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.12%3A_Energy_Level.txt
How is it that so many planes are able to fly without running into each other? The flight path of a commercial airliner is carefully regulated by the Federal Aviation Administration. Each airplane must maintain a distance of five miles from another plane flying at the same altitude, and be 2,000 feet above and below another aircraft (1,000 feet if the altitude is less than 29,000 feet). So, each aircraft only has certain positions it is allowed to maintain while it flies. Quantum mechanics demonstrates that electrons have similar restrictions on their locations. Orbitals We can apply our knowledge of quantum numbers to describe the arrangement of electrons for a given atom. We do this with something called electron configurations. They are effectively a map of the electrons for a given atom. We look at the four quantum numbers for a given electron and then assign that electron to a specific orbital. $s$ Orbitals For any value of $n$, a value of $l=0$ places that electron in an $s$ orbital. This orbital is spherical in shape: $p$ Orbitals From the table below, it is evident that there are three possible orbitals when $l=1$. These are designated as $p$ orbitals and have dumbbell shapes. Each of the $p$ orbitals has a different orientation in three-dimensional space. $d$ Orbitals When $l=2$, $m_l$ values can be $-2, \: -1, \: 0, \: +1, \: +2$ for a total of five $d$ orbitals. Note that all five of the orbitals have specific three-dimensional orientations. $f$ Orbitals The most complex set of orbitals are the $f$ orbitals. When $l=3$, $m_l$ values can be $-3, \: -2, \: -1, \: 0, \: +1, \: +2, \: +3$ for a total of seven different orbital shapes. Again, note the specific orientations of the different $f$ orbitals. Table $1$: Electron Arrangement Within Energy Levels Principal Quantum Number $\left( n \right)$ Allowable Sublevels Number of Orbitals per Sublevel Number of Orbitals per Principal Energy Level Number of Electrons per Sublevel Number of Electrons per Principal Energy Level 1 $s$ 1 1 2 2 2 $s$ 1 4 2 8 $p$ 3 6 3 $s$ 1 9 2 18 $p$ 3 6 $d$ 5 10 4 $s$ 1 16 2 32 $p$ 3 6 $d$ 5 10 $f$ 7 14 Summary • There are four different classes of electron orbitals. • Electron orbitals are determined by the value of the angular momentum quantum number $l$. Review 1. What is an electron configuration? 2. How many electrons are in the $n=1$ orbital? 3. What is the total number of electrons in a $p$ orbital? 4. How many electrons does it take to completely fill a $d$ orbital? 5.14: Quantum Numbers Can you guess how many people are in this stadium? If you attend a college or professional football game, you need a ticket to get in. It is very likely that your ticket may specify a gate number, section number, row, and seat number. No other ticket can have the same four parts to it. It may even have the same gate, section, and row, but it would still have to include a different seat number. Each seat is unique and allows only one occupant to fill it. Quantum Numbers We use a series of specific numbers, called quantum numbers, to describe the location of an electron in an associated atom. Quantum numbers specify the properties of the atomic orbitals and the electrons in those orbitals. An electron in an atom or ion has four quantum numbers to describe its state. Think of them as important variables in an equation that describe the three-dimensional position of electrons in a given atom. Principal Quantum Number $\left( n \right)$ The principal quantum number, signified by $n$, is the main energy level occupied by the electron. Energy levels are fixed distances from the nucleus of a given atom. They are described in whole number increments (e.g., 1, 2, 3, 4, 5, 6, ...). At location $n=1$, an electron would be closest to the nucleus, while at $n=2$ the electron would be farther, and at $n=3$ farther yet. As we will see, the principal quantum number corresponds to the row number for an atom on the periodic table. Angular Momentum Quantum Number $\left( l \right)$ The angular momentum quantum number, signified by $l$, describes the general shape or region an electron occupies—its orbital shape. The value of $l$ depends on the value of the principal quantum number, $n$. The angular momentum quantum number can have positive values of zero to $\left( n-1 \right)$. If $n=2$, $l$ could be either $0$ or $1$. Magnetic Quantum Number $\left( m_l \right)$ The magnetic quantum number, signified as $m_l$, describes the orbital orientation in space. Electrons can be situated in one of three planes in three dimensional space around a given nucleus ($x$, $y$, and $z$). For a given value of the angular momentum quantum number, $l$, there can be $\left( 2l+1 \right)$ values for $m_l$. As an example: $n=2$ $l=0$ or $1$ for $l=0$, $m_l = 0$ for $l=1$, $m_l = -1, 0, +1$ Table $1$: Principal Energy Levels and Sublevels Principal Energy Level Number of Possible Sublevels Possible Angular Momentum Quantum Numbers Orbital Designation by Principal Energy Level and Sublevel $n=1$ 1 $l=0$ $1s$ $n=2$ 2 $l=0$ $2s$ $l=1$ $2p$ $n=3$ 3 $l=0$ $3s$ $l=1$ $3p$ $l=2$ $3d$ $n=4$ 4 $l=0$ $4s$ $l=1$ $4p$ $l=2$ $4d$ $l=3$ $4f$ The table above shows the possible angular momentum quantum number values $\left( l \right)$ for the corresponding principal quantum numbers $\left( n \right) of \(n = 1, n=2, n=3,$ and $n=4$. Spin Quantum Number $\left( m_s \right)$ The spin quantum number describes the spin for a given electron. An electron can have one of two associated spins, $\left( + \frac{1}{2} \right)$ spin, or $\left( -\frac{1}{2} \right)$ spin. An electron cannot have zero spin. We also represent spin with arrows $\uparrow$ or $\downarrow$. A single orbital can hold a maximum of two electrons, and each must have opposite spin. Summary • Quantum numbers specify the arrangements of electrons in orbitals. • There are four quantum numbers that provide information about various aspects of electron behavior. Review 1. What do quantum numbers do? 2. What is the principal quantum number? 3. What does the spin quantum number represent? 5.15: Aufbau Principle How are buildings constructed? Construction of a building begins at the bottom. The foundation is laid, and the building goes up step by step. You obviously cannot start with the roof, since there is no place to hang it. The building goes from the lowest level to the highest level in a systematic way. Aufbau Principle In order to create ground state electron configurations for any element, it is necessary to know the way in which the atomic sublevels are organized in order of increasing energy. Figure $2$ shows the order of increasing energy of the sublevels. The lowest energy sublevel is always the $1s$ sublevel, which consists of one orbital. The single electron of the hydrogen atom will occupy the $1s$ orbital when the atom is in its ground state. As we proceed to atoms with multiple electrons, those electrons are added to the next lowest sublevel: $2s$, $2p$, $3s$, and so on. The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest. The Aufbau (German for building up, construction) principle is sometimes referred to as the "building up" principle. It is worth noting that in reality, atoms are not built by adding protons and electrons one at a time, and that this method is merely an aid to understand the end result. As seen in the figure above, the energies of the sublevels in different principal energy levels eventually begin to overlap. After the $3p$ sublevel, it would seem logical that the $3d$ sublevel should be the next lowest in energy. However, the $4s$ sublevel is slightly lower in energy than the $3d$ sublevel and thus fills first. Following the filling of the $3d$ sublevel is the $4p$, then the $5s$ and the $4d$. Note that the $4f$ sublevel does not fill until just after the $6s$ sublevel. Figure $2$ is a useful and simple aid for keeping track of the order of fill of the atomic sublevels. Summary • The Aufbau principle gives the order of electron filling in an atom. • It can be used to describe the locations and energy levels of every electron in a given atom. Review 1. What is the Aufbau principle? 2. Which orbital is filled after the $2p$? 3. Which orbital is filled after $4s$? 4. Which orbital is filled after $6s$?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.13%3A_Orbitals.txt
Can you name one thing that easily distinguishes you from the rest of the world? And we're not talking about DNA—that's a little expensive to sequence. For many people, it is their email address. My email address allows people all over the world to contact me. It does not belong to anyone else, but serves to identify me. Electrons also have a unique set of identifiers in the quantum numbers that describe their location and spin. Pauli Exclusion Principle When we look at the orbital possibilities for a given atom, we see that there are different arrangements of electrons for each different type of atom. Since each electron must maintain its unique identity, we intuitively sense that the four quantum numbers for any given electron must not match up exactly with the four quantum numbers for any other electron in that atom. For the hydrogen atom, there is no problem since there is only one electron in the $\ce{H}$ atom. However, for helium, we see that the first three quantum numbers for the two electrons are the same: same energy level, same spherical shape. What differentiates the two helium electrons is their spin. One of the electrons has a $+\frac{1}{2}$ spin while the other electron has a $-\frac{1}{2}$ spin. So, the two electrons in the $1s$ orbital are each unique and distinct from one another because their spins are different. This observation leads to the Pauli exclusion principle, which states that no two electrons in an atom can have the same set of four quantum numbers. The energy of the electron is specified by the principal, angular momentum, and magnetic quantum numbers. If those three numbers are identical for two electrons, the spin numbers must be different in order for the two electrons to be differentiated from one another. The two values of the spin quantum number allow each orbital to hold two electrons. Figure $2$ shows how the electrons are indicated in a diagram. Summary • The Pauli exclusion principle specifies limits on how identical quantum numbers can be for two electrons in the same atom. Review 1. What is the difference between the two helium electrons? 2. What does the Pauli exclusion principle state? 3. What does the two values for the spin quantum number allow? 5.17: Hund's Rule and Orbital Filling Diagrams Have you ever wondered what those load limit signs mean on a bridge? The sign above signifies that nothing over five tons is allowed because it will do damage to the structure. There are limits to the amount of weight that a bridge can support, there are limits to the number of people that can safely occupy a room, and there are limits to what can go into an electron orbital. Hund's Rule The last of the three rules for constructing electron arrangements requires electrons to be placed one at a time in a set of orbitals within the same sublevel. This minimizes the natural repulsive forces that one electron has for another. Hund's rule states that orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and that each of the single electrons must have the same spin. The figure below shows how a set of three $p$ orbitals is filled with one, two, three, and four electrons. Orbital Filling Diagrams An orbital filling diagram is the more visual way to represent the arrangement of all the electrons in a particular atom. In an orbital filling diagram, the individual orbitals are shown as circles (or squares) and orbitals within a sublevel are drawn next to each other horizontally. Each sublevel is labeled by its principal energy level and sublevel. Electrons are indicated by arrows inside the circles. An arrow pointing upwards indicates one spin direction, while a downward pointing arrow indicates the other direction. The orbital filling diagrams for hydrogen, helium, and lithium are shown in the figure below. According to the Aufbau process, sublevels and orbitals are filled with electrons in order of increasing energy. Since the $s$ sublevel consists of just one orbital, the second electron simply pairs up with the first electron as in helium. The next element is lithium and necessitates the use of the next available sublevel, the $2s$. The filling diagram for carbon is shown in the figure below. There are two $2p$ electrons for carbon and each occupies its own $2p$ orbital. Oxygen has four $2p$ electrons. After each $2p$ orbital has one electron in it, the fourth electron can be placed in the first $2p$ orbital with a spin opposite that of the other electron in that orbital. Summary • Hund's rule specifies the order of electron filling within a set of orbitals. • Orbital filling diagrams are a way of indicating electron locales in orbitals. Review 1. State Hund’s rule. 2. What is an orbital filling diagram? 3. Is the diagram in figure below correct? Explain your answer. 4. Is the diagram in figure below correct? Explain your answer.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.16%3A_Pauli_Exclusion_Principle.txt
How big is a file? If you keep your papers in manila folders, you can pick up a folder and see how much it weighs. If you want to know how many different papers (articles, bank records, or whatever else you keep in a folder), you have to take everything out and count. A computer directory, on the other hand, tells you exactly how much you have in each file. We can get the same information on atoms. If we use an orbital filling diagram, we have to count arrows. When we look at electron configuration data, we simply add up the numbers. Electron Configurations Electron configuration notation eliminates the boxes and arrows of orbital filling diagrams. Each occupied sublevel designation is written followed by a superscript that is the number of electrons in that sublevel. For example, the hydrogen configuration is $1s^1$, while the helium configuration is $1s^2$. Multiple occupied sublevels are written one after another. The electron configuration of lithium is $1s^2 2s^1$. The sum of the superscripts in an electron configuration is equal to the number of electrons in that atom, which is in turn equal to its atomic number. Example $1$ Draw the orbital filling diagram for carbon and write its electron configuration. Known • atomic number of carbon, Z=6 Use the order of fill diagram to draw an orbital filling diagram with a total of six electrons. Follow Hund's rule. Write the electron configuration. Step 2: Construct diagram. Electron configuration $1s^2 2s^2 2p^2$ Step 3: Think about your result. Following the $2s$ sublevel is the $2p$, and $p$ sublevels always consist of three orbitals. All three orbitals need to be drawn even if one or more is unoccupied. According to Hund's rule, the sixth electron enters the second of those $p$ orbitals, with the same spin as the fifth electron. Second Period Elements Periods refer to the horizontal rows of the periodic table. The first period of the periodic table contains only the elements hydrogen and helium. This is because the first principal energy level consists of only the $s$ sublevel, and so only two electrons are required in order to fill the entire principal energy level. Each time a new principal energy level begins, as with the third element lithium, a new period is started on the periodic table. As one moves across the second period, electrons are successively added. With beryllium $\left( Z=4 \right)$, the $2s$ sublevel is complete and the $2p$ sublevel begins with boron $\left( Z=5 \right)$. Since there are three $2p$ orbitals and each orbital holds two electrons, the $2p$ sublevel is filled after six elements. Table $1$ shows the electron configurations of the elements in the second period. Element Name Symbol Atomic Number Electron Configuration Table $1$: Electron Configurations of Second-Period Elements Lithium $\ce{Li}$ 3 $1s^2 2s^1$ Beryllium $\ce{Be}$ 4 $1s^2 2s^2$ Boron $\ce{B}$ 5 $1s^2 2s^2 2p^1$ Carbon $\ce{C}$ 6 $1s^2 2s^2 2p^2$ Nitrogen $\ce{N}$ 7 $1s^2 2s^2 2p^3$ Oxygen $\ce{O}$ 8 $1s^2 2s^2 2p^4$ Fluorine $\ce{F}$ 9 $1s^2 2s^2 2p^5$ Neon $\ce{Ne}$ 10 $1s^2 2s^2 2p^6$ Summary • Electron configuration notation simplifies the indication of where electrons are located in a specific atom. • Superscripts are used to indicate the number of electrons in a given sublevel. Review 1. What does electron configuration notation eliminate? 2. How do we know how many electrons are in each sublevel? 3. An atom has the electron configuration of $1s^2 2s^2 2p^5$. How many electrons are in that atom? 4. Which element has the electron configuration of $1s^2 2s^2 2p^6 3s^2$?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.18%3A_Electron_Configurations.txt
What makes a particular element very reactive and another element non-reactive? A chemical reaction involves either electron removal, electron addition, or electron sharing. The path that a specific element will take in a reaction depends on where the electrons are in the atom and how many there are. Valence Electrons In the study of chemical reactivity, electrons in the outermost principal energy level are very important and so are given a special name. Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the $1s$ sublevel are called inner-shell electrons and are not involved directly in the element's reactivity, or in the formation of compounds. Lithium has a single electron in the second principal energy level, and so we say that lithium has one valence electron. Beryllium has two valence electrons. How many valence electrons does boron have? Recognize that the second principal energy level consists of both the $2s$ and the $2p$ sublevels, and so the answer is three. In fact, the number of valence electrons goes up by one for each step across a period, until the last element is reached. Neon, with its configuration ending in $2s^2 2p^6$, has eight valence electrons. Summary • Valence electrons are the outer-shell electrons of an atom. • Valence electrons determine the reactivity of an atom. Review 1. Define valence electron. 2. Define inner shell electron. 3. How many valence electrons are there in fluorine? 4. What are the 2s electrons in nitrogen? 5. How many inner shell electrons are there in beryllium? 5.20: Noble Gas Configuration How does it feel to be full after a meal? Envision that you have nearly finished a great meal, but cannot put another bite in your mouth because there is no place for it to go. The noble gases have the same problem—there is no room for any more electrons in their outer shells. They are completely full and cannot handle any more. Noble Gas Configuration Sodium, element number 11, is the first element in the third period of the periodic table. Its electron configuration is $1s^2 2s^2 2p^6 3s^1$. The first ten electrons of the sodium atom are the inner-shell electrons and the configuration of just those ten electrons is exactly the same as the configuration of the element neon $\left( Z=10 \right)$. This provides the basis for a shorthand notation for electron configurations called the noble gas configuration. The elements that are found in the last column of the periodic table are an important group of elements called the noble gases. They are helium, neon, argon, krypton, xenon, and radon. A noble gas configuration of an atom consists of the elemental symbol of the last noble gas prior to that atom, followed by the configuration of the remaining electrons. So for sodium, we make the substitution of $\left[ \ce{Ne} \right]$ for the $1s^2 2s^2 2p^6$ part of the configuration. Sodium's noble gas configuration becomes $\left[ \ce{Ne} \right] 3s^1$. Table $1$ shows the noble gas configurations of the third period elements. Table $1$: Electron Configurations of Third-Period Elements Element Name Symbol Atomic Number Noble Gas Electron Configuration Sodium $\ce{Na}$ 11 $\left[ \ce{Ne} \right] 3s^1$ Magnesium $\ce{Mg}$ 12 $\left[ \ce{Ne} \right] 3s^2$ Aluminum $\ce{Al}$ 13 $\left[ \ce{Ne} \right] 3s^2 3p^1$ Silicon $\ce{Si}$ 14 $\left[ \ce{Ne} \right] 3s^2 3p^2$ Phosphorus $\ce{P}$ 15 $\left[ \ce{Ne} \right] 3s^2 3p^3$ Sulfur $\ce{S}$ 16 $\left[ \ce{Ne} \right] 3s^2 3p^4$ Chlorine $\ce{Cl}$ 17 $\left[ \ce{Ne} \right] 3s^2 3p^5$ Argon $\ce{Ar}$ 18 $\left[ \ce{Ne} \right] 3s^2 3p^6$ Again, the number of valence electrons increases from one to eight across the third period. The fourth and subsequent periods follow the same pattern, except for the use of a different noble gas. Potassium has nineteen electrons, one more than the noble gas argon, so its configuration could be written as $\left[ \ce{Ar} \right] 4s^1$. In a similar fashion, strontium has two more electrons than the noble gas krypton, which would allow us to write its electron configuration as $\left[ \ce{Kr} \right] 5s^2$. All elements can be represented in this fashion. Summary • The noble gas configuration system allows some shortening of the total electron configuration by using the symbol for the noble gas of the previous period as part of the pattern of electrons. Review 1. What is the element represented by $\left[ \ce{Ne} \right] 3s^2 3p^2$? 2. What element has this electron configuration $\left[ \ce{Ar} \right] 3d^7 4s^2$? 3. What noble gas would be part of the electron configuration notation for Mn? 4. How would you write the electron configuration for Ba?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/05%3A_Electrons_in_Atoms/5.19%3A_Valence_Electrons.txt
The modern periodic table emphasizes the electronic structure of atoms. The original periodic table emphasized the reactivity of the elements. In this chapter we will learn about the connection between the two, and how the electronic structure is related to the macroscopic properties of the elements. 06: The Periodic Table When you go to the library to find a book, how do you locate it? If it is a fiction book, you look by author since the fiction materials are filed by the author’s last name. If you are looking for a non-fiction publication, you look in a catalog (most likely on a computer these days). The book you are looking for will have a number by the title. This number refers to the Dewey Decimal system, developed by Melvil Dewey in 1876 and used in over 200,000 libraries throughout the world. Another system in wide use is the Library of Congress approach, developed in the late 1800s-early 1900s to organize the materials in the federal Library of Congress. This method is one of the most widely used ways to organize libraries in the world. Both approaches organize information so that people can easily find what they are looking for. Chemistry information also needs to be organized so we can see patterns of properties in elements. Early Attempts to Organize Elements By the year 1700, only a handful of elements had been identified and isolated. Several of these, such as copper and lead, had been known since ancient times. As scientific methods improved, the rate of discovery dramatically increased. With the ever-increasing number of elements, chemists recognized that there may be some kind of systematic way to organize the elements. The question was: how? A logical way to begin grouping elements together was by their chemical properties. (In other words, putting elements in separate groups based on how they reacted with other elements.) In 1829, a German chemist, Johann Dobereiner (1780-1849), placed various groups of three elements into groups called triads. One such triad was lithium, sodium, and potassium. Triads were based on both physical, as well as chemical, properties. Dobereiner found that the atomic masses of these three elements, as well as other triads, formed a pattern. When the atomic masses of lithium and potassium were averaged together $\left( \frac{\left( 6.94 + 39.10 \right)}{2} = 23.02 \right)$, it was approximately equal to the atomic mass of sodium (22.99). These three elements also displayed similar chemical reactions, such as vigorously reacting with the members of another triad: chlorine, bromine, and iodine. While Dobereiner's system would pave the way for future ideas, a limitation of the triad system was that not all of the known elements could be classified in this way. English chemist John Newlands (1838-1898) ordered the elements in increasing order of atomic mass and noticed that every eighth element exhibited similar properties. He called this relationship the "Law of Octaves". Unfortunately, there were some elements that were missing and the law did not seem to hold for elements that were heavier than calcium. Newlands' work was largely ignored and even ridiculed by the scientific community in his day. It was not until years later that another more extensive periodic table effort would gain much greater acceptance, and that the pioneering work of John Newlands would be appreciated. Summary • Johann Dobereiner organized elements into groups called triads. • John Newlands proposed the "Law of Octaves" for organizing the elements. Review 1. List some elements known since ancient times? 2. What properties were the basis of the triad system? 3. Why did Dobereiner believe that lithium, sodium, and potassium belonged in a triad? 4. What was a shortcoming of the triad system? 5. How did Newlands arrange the element? 6. What was a problem with the “Law of Octaves"?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.01%3A_Early_History_of_the_Periodic_Table.txt
When you study for a test, how do you approach the task? One useful way to study for a test is to use flashcards. To make them, write down vocabulary words, foreign language terms, math formulas, chemistry reactions—anything you want to learn. Then sort these cards into categories and topics that go together. This organization of information helps you see patterns in the material so that you can tie different ideas together and make better sense of them. The periodic table was first built using a set of cards. With this strategy, Dmitri Mendeleev, the creator of the periodic table, could organize and rearrange material until patterns emerged. Mendeleev's Periodic Table In 1869, Russian chemist and teacher Dmitri Mendeleev (1836-1907) published a periodic table of the elements. The following year, German chemist Lothar Meyer independently published a very similar table. Mendeleev is generally given more credit than Meyer because his table was published first, and because of several key insights that he made regarding the table. Mendeleev was writing a chemistry textbook for his students and wanted to organize all of the known elements at that time according to their chemical properties. He famously organized the information for each element onto separate note cards that were then easy to rearrange as needed. He discovered that when he placed them in order of increasing atomic mass, certain similarities in chemical behavior repeated at regular intervals. This type of a repeating pattern is called "periodic". A pendulum that swings back and forth in a given time interval is periodic, as is the movement of the moon around the Earth. In the figure above, atomic mass increases from top to bottom of vertical columns, with successive columns going left to right. As a result, elements that are in the same horizontal row are groups of elements that were known to exhibit similar chemical properties. One of Mendeleev's insights is illustrated by the elements tellurium $\left( \ce{Te} \right)$ and iodine $\left( \ce{I} \right)$. Notice that tellurium is listed before iodine even though its atomic mass is higher. Mendeleev reversed the order because he knew that the properties of iodine were much more similar to those of fluorine $\left( \ce{F} \right)$, chlorine $\left( \ce{Cl} \right)$, and bromine $\left( \ce{Br} \right)$ than they were to oxygen $\left( \ce{O} \right)$, sulfur $\left( \ce{S} \right)$, and selenium $\left( \ce{Se} \right)$. He simply assumed that there was an error in the determination of one or both of the atomic masses. This turned out not to be the case, but Mendeleev was indeed correct to group these two elements as he did. Notice that there are several places in the table that have no chemical symbol, but are instead labeled with a question mark. Between zinc $\left( {Zn} \right)$ and arsenic $\left( \ce{As} \right)$ are two such missing elements. Mendeleev believed that elements with atomic masses of 68 and 70 would eventually be discovered and that they would fit chemically into each of those spaces. Listed in the table below are other properties that Mendeleev predicted for the first of these two missing elements, which he called "eka-aluminum", compared with the element gallium. Mendeleev's predicted properties for Eka-aluminum and gallium. Eka-Aluminum $\left( \ce{Ea} \right)$ Gallium $\left( \ce{Ga} \right)$ Atomic Mass $68 \: \text{amu}$ $69.9 \: \text{amu}$ Melting Point Low $30.15^\text{o} \text{C}$ Density $5.9 \: \text{g/cm}^3$ $5.94 \: \text{g/cm}^3$ Formula of Oxide $\ce{Ea_2O_3}$ $\ce{Ga_2O_3}$ The element gallium was discovered four years after the publication of Mendeleev's table, and its properties matched up remarkably well with eka-aluminum, fitting into the table exactly where he had predicted. This was also the case with the element that followed gallium, which was eventually named germanium. Mendeleev's periodic table gained wide acceptance with the scientific community and earned him credit as the discoverer of the periodic law. Element number 101, synthesized in 1955, is named mendelevium after the founder of the periodic table. It was, however, several years after Mendeleev died before the several discrepancies with the atomic masses could be explained, and before the reasons behind the repetition of chemical properties could be fully explained. Summary • Mendeleev published his periodic table in 1869. • His organization of elements was based on atomic mass. • Mendeleev's periodic table made it possible to predict properties of elements that had not yet been discovered. Review 1. When did Mendeleev publish his periodic table? 2. Who else came out with a periodic table at about the same time? 3. Why was Mendeleev’s table considered to be superior? 4. What element did Mendeleev predict to exist? 5. What element was named after Mendeleev? 6.03: Periodic Law How are these items related to one another? We have all enjoyed looking around a market for delicious foods to eat later at home. When you get to the market you know you need to get fruits, vegetables and grains to ensure you eat a balanced diet. In the market, these items are all grouped together to make it easier to find the type of fruit, vegetable or grain you are looking for; it would be inconvenient if the different fruits were all scattered in different places. The periodic table is organized in a similar way, ensuring similar elements are found in the same group or period. The Periodic Law When Mendeleev put his periodic table together, nobody knew about the existence of the nucleus. It was not until 1911 that Rutherford conducted his gold foil experiment that demonstrated the presence of the nucleus in the atom. Just two years later, in 1913, English physicist Henry Moseley (1887-1915) examined x-ray spectra of a number of chemical elements. He would shoot x-rays through crystals of the element and study the wavelengths of the radiation that he detected. Moseley found that there was a relationship between wavelength and atomic number. His results led to the definition of atomic number as the number of protons contained in the nucleus of each atom. He then realized that the elements of the periodic table should be arranged in order of increasing atomic number, rather than increasing atomic mass. When ordered by atomic number, the discrepancies within Mendeleev's table disappeared. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law: When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups. Summary • Elements of the periodic table are arranged in order of increasing atomic number. • The periodic law states: "When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties." Review 1. Did Mendeleev know about the nucleus of an atom? 2. Who discovered the relationship between wavelength of X-rays and atomic number? 3. What did Moseley conclude from his research? 4. What is the “periodic law”? 5. What do the vertical columns (groups) in the periodic table represent?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.02%3A_Mendeleev%27s_Periodic_Table.txt
How has the English dictionary evolved over time? Language changes with time. New words enter the language and old words often disappear from lack of use. Dictionaries are published so that people can keep up with changes in language and know how to use words properly. These publications may be in print, as is the law dictionary below, or they may be electronic. Dictionaries can be found on the internet and apps are available for smartphones. Dictionaries are invaluable for good, reliable communication. The Modern Periodic Table The periodic table has undergone extensive changes in the time since it was originally developed by Mendeleev and Moseley. Many new elements have been discovered, while others have been artificially synthesized. Each fits properly into a group of elements with similar properties. The periodic table is an arrangement of the elements in order of their atomic numbers, so that elements with similar properties appear in the same vertical column or group. The figure below shows the most commonly used form of the periodic table. Each square shows the chemical symbol of the element along with its name. Notice that several of the symbols seem to be unrelated to the name of the element: $\ce{Fe}$ for iron, $\ce{Pb}$ for lead, etc. Most of these are the elements that have been known since ancient times and have symbols based on their Latin names. The atomic number of each element is written above the symbol. A period is a horizontal row of the periodic table. There are seven periods in the periodic table, with each one beginning at the far left. A new period begins when a new principal energy level begins filling with electrons. Period 1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements. Periods 4 and 5 have 18 elements. Periods 6 and 7 have 32 elements, because the two bottom rows that are separate from the rest of the table belong to those periods. These two rows are pulled out in order to make the table itself fit more easily onto a single page. A group is a vertical column of the periodic table, based on the organization of the outer shell electrons. There are a total of 18 groups. There are two different numbering systems that are commonly used to designate groups, and you should be familiar with both. The traditional system used in the United States involves the use of the letters A and B. The first two groups are 1A and 2A, while the last six groups are 3A through 8A. The middle groups use B in their titles. Unfortunately, there was a slightly different system in place in Europe. To eliminate confusion, the International Union of Pure and Applied Chemistry (IUPAC) decided that the official system for numbering groups would be a simple 1 through 18 from left to right. Many periodic tables show both systems simultaneously. Most recent depictions of the periodic table show an incomplete seventh period. In fact, many of those seventh period elements were not known before the early twentieth century. Many elements have been synthesized by bombarding known elements with sub-atomic particles such as neutrons or alpha particles. Uranium has been used in this manner to produce elements 93-100. Larger elements (atomic numbers 101 and above) are formed by fusing nuclei of smaller elements together. These synthetic elements tend to be very unstable, often existing for less than a second, so little is known about them. Elements up to atomic number 112 are known (not in any detail, however), and some evidence for elements 113 and above had been put forth, with elements 114 and 116 being added to the table in 2011. In December 2015, the IUPAC verified the existence of four new elements 113, 115, 117, and 118 and approved their addition to the periodic table. The following are the new element names and their origins: • Element 113 was named Nihonium, symbol Nh, proposed by Japanese researchers after the Japanese word Nihon, which means Japan. • A team of scientists from Russia and the United States named element 115, Moscovium, symbol Mc, after Moscow and element 117, Tennessine, symbol Ts, after Tennessee. • The Russian team that discovered element 118 named it Oganesson, symbol Og, after Yuri Oganessian, a prolific element hunter. These elements complete the seventh period or row of the periodic table. The video below reviews groups and trends in the periodic table: Watch the additional videos below to learn more about the newly identified and named chemical elements: Element Name and Symbol: Nihonium (Nh) Atomic Number: 113 Element Name and Symbol: Moscovium (Mc) Atomic Number: 115 Element Name and Symbol: Tennessine (Ts) Atomic Number: 117 Element Name and Symbol: Oganesson (Og) Atomic Number: 118 Summary • The periodic table is arranged in order of atomic number. • A period is a horizontal row of the periodic table. • A group is a vertical row of the periodic table. Review 1. How is today’s periodic table different from the one that Mendeleev published? 2. Are all the elements in today’s periodic table naturally occurring? Explain your answer. 3. What is a “period"? What does it represent? 4. What is a “group"? What does it represent? 5. Why are there two different numbering systems for groups?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.04%3A_Modern_Periodic_Table-_Periods_and_Groups.txt
Can you guess what types of metals these screws are made of? Screws come in all sizes and shapes. They are all (well, almost all) made of some kind of metal. But they have differences in size, shape, and type of metal. Physical characteristics also differ. Some screws are long, and others are short. One screw may have a flat-head slot while another screw may have a Phillips-head. Some of the screws in the picture below are used to fasten things together, and others are used to hang heavy objects on a wall. Chemists classify materials in many ways. We can sort elements on the basis of their electron arrangements. The way the electrons are distributed determines the chemical properties of the element. Another way is to classify elements based on physical properties. Some common physical properties are color, volume, and density. Other properties that allow us to sort on the basis of behavior are conduction of heat and electricity, malleability (the ability to be hammered into very thin sheets), ductility (the ability to be pulled into thin wires), melting point, and boiling point. Three broad classes of elements based on physical properties are metals, nonmetals, and metalloids. Metals A metal is an element that is a good conductor of heat and electricity. Metals are also malleable, which means that they can be hammered into very thin sheets without breaking. They are ductile, which means that they can be drawn into wires. When a fresh surface of any metal is exposed, it will be very shiny because it reflects light well. This is called luster. All metals are solid at room temperature with the exception of mercury (Hg), which is a liquid. Melting points of metals display a very wide variance. The melting point of mercury is -39°C, while the highest melting metal is tungsten (W), with a melting point of 3422°C. The elements in blue in the periodic table below are metals. About 80 percent of the elements are metals. Gold has been used by many civilizations for making jewelry (see figure below). This metal is soft and easily shaped into a variety of items. Since gold is very valuable and often used as currency, gold jewelry has also often represented wealth. Copper is a good conductor of electricity and is very flexible and ductile. This metal is widely used to conduct electric current in a variety of appliances, from lamps to stereo systems to complex electronic devices (see figure below). Mercury is the only metal to exist as a liquid at room temperature (see figure below). This metal was extensively used in thermometers for decades until information about its toxicity became known. Mercury switches were once common, but are no longer used. However, new federally-mandated energy-efficient light bulbs that are now used contain trace amounts of mercury and represent a hazardous waste. Summary • Metals are good conductors of heat and electricity. • Metals are malleable and ductile • All metals are solids at room temperature with the exception of mercury • Gold, silver, iron, and mercury are typical metals. Review 1. What properties of an element are affected by electron distribution? 2. Define malleability. 3. Define ductility. 4. State one reason gold is used in jewelry. 5. Why is mercury no longer used in many devices?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.05%3A_Metals.txt
When we sort parts in our shop or garage, we often classify them in terms of common properties. One container might hold all the screws (possibly sub-divided by size and type). Another container would be for nails. Maybe there is a set of drawers for plumbing parts. When you get finished, you could also have a collection of things that don’t nicely fit a category. You define them in terms of what they are not. They are not electrical components, or sprinkler heads for the yard, or parts for the car. These parts may have some common properties, but are a variety of items. Nonmetals In the chemical world, these “spare parts” would be considered nonmetals, loosely defined as not having the properties of metals. A nonmetal is an element that is generally a poor conductor of heat and electricity. Most properties of nonmetals are the opposite of metals. There is a wider variation in properties among the nonmetals than among the metals. Nonmetals exist in all three states of matter. The majority are gases, such as nitrogen and oxygen. Bromine is a liquid. A few are solids, such as carbon and sulfur. In the solid state, nonmetals are brittle, meaning that they will shatter if struck with a hammer. The solids are not lustrous. Melting points are generally much lower than those of metals. The green elements in the table below are nonmetals. Nonmetals have a wide variety of uses. Sulfur can be employed in gunpowder, fireworks, and matches to facilitate ignition (see Figure below). This element is also widely used as an insecticide, a fumigant, or a means of eliminating certain types of fungus. An important role for sulfur is the manufacture of rubber for tires and other materials. First discovered in 1839 by Charles Goodyear, the process of vulcanization makes the rubber more flexible and elastic as well as being more resistant to changes in temperature. A major use of sulfur is for the preparation of sulfur-containing compounds such as sulfuric acid. Bromine is a versatile compound, used mainly in manufacture of flame-retardant materials, especially important for children’s clothing (see Figure below). For treatment of water in swimming pools and hot tubs, bromine is beginning to replace chlorine as a disinfectant because of its higher effectiveness. When incorporated into compounds, bromine atoms play important roles in pharmaceuticals for treatment of pain, cancer, and Alzheimer’s disease. Helium is one of the many nonmetals that is a gas. Other nonmetal gases include hydrogen, fluorine, chlorine, and all the group eighteen noble (or inert) gases. Helium is chemically non-reactive, so it is useful for applications such as balloons (see figure below) and lasers, where non-flammability is extremely important. Liquid helium exists at an extremely low temperature and can be used to cool superconducting magnets for imaging studies (MRI, magnetic resonance imaging). Leaks in vessels and many types of high-vacuum apparatus can be detected using helium. Inhaling helium changes the speed of sound, producing a higher pitch in your voice. This is definitely an unsafe practice and can lead to physical harm and death. Summary • Nonmetals are generally poor conductors of heat and electricity. • Properties of nonmetals are usually the opposite of properties of metals • Nonmetals can be solid, liquid, or gas at room temperature depending upon the element. • Sulfur, bromine, and helium are typical nonmetals. Review 1. What are the properties of nonmetals? 2. List the states of matter in which nonmetals can exist and give one example of each state. 3. What are the physical properties and uses of sulfur? 4. What are the physical properties and uses of bromine? 5. What are the physical properties and uses of helium? 6.07: Metalloids Have you ever taken a multiple-choice test? Most of the time the answers are specific choices – is the answer possibility a or possibility b? Quite often you can “think through” the choices to come up with the correct answer. More frustrating is the choice “none of the above.” You feel very uncertain checking that possibility. Metalloids Some elements are “none of the above.” They don’t fit neatly into the categories of metal or non-metal because of their characteristics. A metalloid is an element that has properties that are intermediate between those of metals and nonmetals. Metalloids can also be called semimetals. On the periodic table, the elements colored yellow, which generally border the stair-step line, are considered to be metalloids. Notice that aluminum borders the line, but it is considered to be a metal since all of its properties are like those of metals. Examples of Metalloids Silicon is a typical metalloid (see figure below). It has luster like a metal, but is brittle like a nonmetal. Silicon is used extensively in computer chips and other electronics because its electrical conductivity is in between that of a metal and a nonmetal. Boron is a versatile element that can be incorporated into a number of compounds (see figure below). Borosilicate glass is extremely resistance to thermal shock. Extreme changes in the temperature of objects containing borosilicates will not create any damage to the material, unlike other glass compositions, which would crack or shatter. Because of their strength, boron filaments are used as light, high-strength materials for airplanes, golf clubs, and fishing rods. Sodium tetraborate is widely used in fiberglass as insulation and also is employed in many detergents and cleaners. Arsenic has long played a role in murder mysteries, being used to commit the foul deed (see figure below). This use of the material is not very smart since arsenic can be easily detected on autopsy. We find arsenic in pesticides, herbicides, and insecticides, but the use of arsenic for these applications is decreasing due to the toxicity of the metal. Its effectiveness as an insecticide has led arsenic to be used as a wood preservative. Antimony is a brittle, bluish-white metallic material that is a poor conductor of electricity (see figure below). Used with lead, antimony increases the hardness and strength of the mixture. This material plays an important role in the fabrication of electronic and semiconductor devices. About half of the antimony used industrially is employed in the production of batteries, bullets, and alloys. Summary • Metalloids are elements with properties intermediate between those of metals and non-metals • Silicon is a metalloid because it has luster, but is brittle. • Boron, arsenic, and antimony are metalloids with a variety of uses. Review 1. Define “metalloid.” 2. Why would it be difficult to decide whether or not an element was a metalloid based on its properties? 3. Why is silicon used extensively in electronics? 4. What are borosilicates used for? 5. Why is the use of arsenic as an insecticide decreasing? 6. What is a main application of antimony?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.06%3A_Nonmetals.txt
What makes these music notes unique? We all enjoy music of some sort. Some people like classical music, others like jazz or country. Music styles change from one period of time to the next, and from one region to another. Each type of music has its language that describes it. Classical music has a certain structure, style, and content. There are different expressions of classical music – the symphony, concerto, sonata. We have ballet and opera as well as choral music. Jazz has a different set of characteristics from classical and different styles of performance. Each type of music can be described and compared to other types on the basis of certain common qualities like notes, chords, and melodic styles. The elements in the periodic table could be considered to be similar to types of music. Each set of elements has its unique set of properties, with different sets of elements having some common characteristics in terms of electron arrangements. We can see patterns of electronic structure and reactivity in the periodic table that allow us to understand better the behavior of individual elements. Periods and Blocks There are seven horizontal rows of the periodic table, called periods. The length of each period is determined by the number of electrons that are capable of occupying the sublevels that fill during that period, as seen in the table below. Period Number of Elements in Period Sublevels in Order of Fill Table $1$: Period Length and Sublevels in the Periodic Table 1 2 $1s$ 2 8 $2s$ $2p$ 3 8 $3s$ $3p$ 4 18 $4s$ $3d$ $4p$ 5 18 $5s$ $4d$ $5p$ 6 32 $6s$ $4f$ $5d$ $6p$ 7 32 $7s$ $5f$ $6d$ $7s$ Recall that the four different sublevels each consist of a different number of orbitals. The $s$ sublevel has one orbital, the $p$ sublevel has three orbitals, the $d$ sublevel has five orbitals, and the $f$ sublevel has seven orbitals. In the first period, only the $1s$ sublevel is being filled. Since all orbitals can hold two electrons, the entire first period consists of just two elements. In the second period, the $2s$ sublevel, with two electrons, and the $2p$ sublevel, with six electrons, are being filled. Consequently, the second period contains eight elements. The third period is similar to the second, filling the $3s$ and $3p$ sublevels. Notice that the $3d$ sublevel does not actually fill until after the $4s$ sublevel. This results in the fourth period containing 18 elements due to the additional 10 electrons that are contributed by the $d$ sublevel. The fifth period is similar to the fourth. After the $6s$ sublevel fills, the $4f$ sublevel with its 14 electrons fills. This is followed by the $5d$ and the $6p$. The total number of elements in the sixth period is 32. The later elements in the seventh period are still being created. So while there is a possible number of 32 elements in the period, the current number is slightly less. The period to which a given element belongs can be easily determined by its electron configuration. For example, consider the element nickel $\left( \ce{Ni} \right)$. Its electron configuration is $\left[ \ce{Ar} \right] \: 3d^8 \: 4s^2$. The highest occupied principal energy level is the fourth, indicated by the 4 in the $4s^2$ portion of the configuration. Therefore, nickel can be found in the fourth period of the periodic table. Based on electron configurations, the periodic table can be divided into blocks denoting which sublevel is in the process of being filled. The $s$, $p$, $d$, and $f$ blocks are illustrated below. The figure also illustrates how the $d$ sublevel is always one principal level behind the period in which that sublevel occurs. In other words, the $3d$ sublevel fills during the fourth period. The $f$ sublevel is always two levels behind. The $4f$ sublevel belongs to the sixth period. Summary • The horizontal rows of the periodic table are called periods. • The length of a period depends on how many electrons are needed to occupy the sublevels that fill the period. • Blocks indicate which sublevel is being filled. Review 1. What are the horizontal rows of the periodic table called? 2. Which sublevel is being filled in period 1? 3. Which sublevel is being filled in period 7? 4. How does the electron configuration of an element give information about the period it is in? 5. What block of elements has the $d$ sublevels being filled?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.08%3A_Blocks_of_the_Periodic_Table.txt
Can you guess what kind of reaction is taking place in this picture? Some chemistry students just enjoy learning about the science, while others are intrigued by the violent reactions that sometimes can occur. Many chemistry classes have been enlivened by the demonstration of how reactive sodium is with water. In some instances, the demonstration has gone off safely. Unfortunately, in other situations students and instructors have incurred serious injury due to their failure to observe proper safety precautions. One value of the periodic table is the ability to make predictions about the behavior of individual elements. By knowing which group an element is in, we can determine the number of reactive electrons and say something about how that element will behave. Hydrogen and Alkali Metals The periodic table is arranged on the basis of atomic numbers (number of protons in the nucleus). One of the valuable consequences of this arrangement is that we can learn a lot about the electron distribution in these atoms. The colors in the table below indicate the different groupings of atoms based on the location and number of electrons in the atom. If we look at Group I (red column), we see that it is labeled alkali metals. Also note the green H above the alkali metals. All of these elements have a similar configuration of outer-shell electrons (see table below). In each case, there is one electron in the outer orbital and that is an s-orbital electron. Hydrogen is not an alkali metal itself, but has some similar properties due to its simple one proton (located in the nucleus), one electron arrangement. The lone electron exists in a s-orbital around the nucleus. For lithium, there are two 1s electrons in an inner orbit and one 2s electron in the outer orbit. The same pattern holds for sodium and potassium. Element, Symbol, Electron Configuration Element Symbol Electron Configuration hydrogen H $\1s^1$ lithium Li $\left[ \ce{He} \right] 2s^1$ sodium Na $\left[ \ce{Ne} \right] 3s^1$ potassium K $\left[ \ce{Ar} \right] 4s^1$ rubidium Rb $\left[ \ce{Kr} \right] 5s^1$ cesium Cs $\left[ \ce{Xe} \right] 6s^1$ francium Fr $\left[ \ce{Rn} \right] 7s^1$ Even an atom with a very complex electron composition such as cesium still has the single $s$ electron in its outer orbital (see figure below). This one electron is very easily removed during chemical reactions. The group I elements react rapidly with oxygen to produce metal oxides. They are very soft metals, which become liquid just above room temperature. Li reacts with water to produce hydrogen gas. Sodium also reacts the same way, just more rapidly. Potassium reacts rapidly with water producing hydrogen gas and heat which ignites the hydrogen gas. Rubidium and cesium react yet more vigorously and explode on contact with water. Summary • Group I (alkali metals and hydrogen) elements all have one electron in their outer shell. This electron is in a $s$ orbital. • The Group I metals are all very reactive with water. Review 1. What group are the alkali metals and hydrogen in? 2. What is the outer shell electron configuration in this group? 3. How reactive are the alkali metals with oxygen? 4. How reactive are these metals with water?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.09%3A_Hydrogen_and_Alkali_Metals.txt
How are oyster shells and chemistry related? We take a lot of chemistry for granted. Very few of us think about the chemistry of bone or oyster shells. Both of these materials have large amounts of calcium compounds in them and play important roles in maintaining the structure of the organism. The shell provides a solid surrounding for the oyster. Bones give support to the body so the person can move around and not just be a soft mass of tissue. Alkaline Earth Metals Group 2 elements are referred to as “alkaline earth” metals (tan column below). The name “alkaline” comes from the fact that compounds of these elements form basic (pH greater than 7) or alkaline solutions when dissolved in water. If the Group 1 elements all have one $s$ electron in their outer orbital, we can predict that the Group 2 elements will have two electrons in that outer shell. The beryllium atom, the first element of Group 2, has an atomic number of four. The atom has the 1s shell filled as well as the 2s shell, giving a total of four electrons ($1s^2 2s^2$). Note that there are two $s$ electrons in the outer shell, a structure that is characteristic of the Group 2 elements. Barium (atomic number 56) has the same outer shell structure of two electrons in the $s$ orbital, even though the internal electron structure for barium is quite complicated. Radium (atomic number 88) has similar properties to barium and is also in the Group 2 category. However, radium is a radioactive element and is generally under the category of radioisotopes in addition to being an alkaline earth metal, because it is not a stable element. The Group 2 elements tend to be less reactive than their Group 1 counterparts. The need to remove two electrons in order for the material to react means more energy is needed for electron removal. However, these elements are reactive enough that they do not exist in their elemental forms in nature, but are present as compounds. Uses of Alkaline Earth Compounds Since magnesium burns brightly, it is used in flares and fireworks. Magnesium alloys with aluminum provide light weight and sturdy materials for airplanes, missiles, and rockets. Several antacids use magnesium hydroxide to neutralize excess stomach acid. Calcium compounds are widely found in limestone, marble, and chalk. Calcium is an important constituent of cement. Other uses include calcium chloride as a deicer and limestone as a white pigment in paints and toothpaste. Strontium is widely used in fireworks and magnets. Barium compounds can be used in paints, filler for rubber, plastic, and resins, and as a contrast medium for X-rays. Many beryllium compounds are toxic, but these materials have been employed in metal alloys. Summary • The alkaline earth elements are in Group 2 of the periodic table. • These elements each have two s electrons in their outer shell. • The alkaline earth elements are less reactive than the alkali metals. Review 1. Why are these elements known as “alkaline earth” elements? 2. How many electrons are in the outer shell of the alkaline earth elements? 3. Are the alkaline earth elements more or less reactive than the alkali metals? Explain your answer. 4. Is radium usually considered as part of the alkaline earth category in terms of chemistry? Explain your answer.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.10%3A_Alkaline_Earth_Metals.txt
What gives these lights their color? Cities at night would be rather boring without all the bright lights. They provide colorful Illuminations and help make things much more visible. We call these lights “neon lights,” but they use several gases to make the different colors. Noble Gases The reactivity of an element can give us important clues as to the electron configuration of that material. If an element is extremely unreactive, this suggests that the electron configuration is such that adding or removing electrons is very unlikely. There must be a stable electron configuration that resists further reaction. The Group 18 (once known as group VIII) elements are essentially chemically inert (light blue column on the right). All these elements exist as monatomic gases at room temperature. If we look at the electron configurations, we see that helium (atomic number 2) has a full shell of two $s$ electrons. Since there are no electrons shielding this shell from the nucleus, these two electrons will be very difficult to remove, making helium unreactive. The remaining elements in the group have full outer shells consisting of two $s$ electrons and six $p$ electrons for an outer shell content of eight electrons. This particular arrangement renders the atoms fairly unreactive. This group has been referred to as the “inert” gases, indicating that they are chemically inert, or unreactive. Another popular term is “noble gases,” suggesting that these gases do not like to have much to do with the other, more common materials (or that they don’t do a lot of work). Noble Gas Compounds In more recent years, a number of reactions using the noble gas elements have been discovered. Although the conventional wisdom was that the complete outer shells of these elements would not allow them to react, some scientists believed that the outer electrons of the larger elements (such as Rn, Xe, and Kr) were far enough away from the nucleus that they should be able to be displaced under the right set of conditions. The first compound (XePtF6) was made with xenon in 1962. Since then, several compounds have been made with radon, xenon, krypton, and argon. Only helium and neon have not formed compounds at this time. Colors of Noble Gases The different gases glow when an electric current is passed through them. Many of these gases are used in displays because of their chemical inertness. They are stable and will not react with other materials in the system. Radon also will give a reddish glow, but is not used because it is radioactive and will not retain its structure as radon for any significant length of time. Royal Society of Chemistry - Colors of the Noble Gases Science Friday: 4850 Feet Below: The Hunt for Dark Matter Why is there a laboratory 4850 feet below the surface? It turns out the depth is a necessary component of the extremely sensitive detector called the Large Underground Xenon experiment. In this video by Science Friday, several scientists explain how this detector helps physicists look for extremely rare particles. Summary • The noble gases are in Group VIII of the periodic table. • Helium has a full outer shell of two $s$ electrons. • The other gases have full outer shells of two $s$ and six $p$ electrons. • Compounds have been formed with Rn, Xe, Kr, and Ar. Review 1. What elements comprise the noble elements? 2. What state are they in at room temperature? 3. Why is helium non-reactive? 4. Why were the other noble gases believed to be non-reactive? 5. When was the first compound formed from xenon? 6. What happens when an electric current is passed through these gases?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.11%3A_Noble_Gases.txt
How do you study a gas that does not exist as such in nature? It’s not as easy as you think. Fluorine is so reactive that we cannot find it free in nature. None of the halogens exist free in nature (unlike some of the metals such as gold and silver) because they are very reactive. The video below shows how violently elemental fluorine reacts with other materials. Halogens Some elements are much more reactive than others. The Group I (red) and Group II (tan) elements can easily lose electrons during a reaction. Elements of other groups are much more likely to accept electrons as they react. The elements of Group VIIA (new Group 17 - fluorine, chlorine, bromine, iodine, and astatine) are called the halogens (yellow column). The term “halogen” means “salt-former” because these elements will readily react with alkali metal and alkaline earth metals to form halide salts. The halogens all have the general electron configuration $ns^2np^5$, giving them seven valence electrons. They are one electron short of having the full outer $s$ and $p$ sublevel, which makes them very reactive. Physical Properties of Halogens As elements, chlorine and fluorine are gases at room temperature, bromine is a dark orange liquid, and iodine is a dark purple-gray solid. Astatine is so rare that its properties are mostly unknown. In the picture below we see chlorine gas on the left (green), bromine solid and vapor in the middle (orange), and solid iodine (grey) on the right. Fluorine is not shown in the picture below because it is too corrosive and will destroy the glass container. None of these elements are found free in nature because of their reactivity. Fluorine is found in combination with cations in several minerals. Chlorine is found in table salt, in the oceans (which are about 2% chloride ion by weight) and in lakes such as the Great Salt Lake in Utah. Small amounts of bromide and iodide salts can be found in the oceans and in brine wells in several states. Watch the following two video experiments of $p$ block elements: This first video is of bromine reacting with aluminum. This second video discusses the properties of halogens and shows a few more reactions they can participate in. Summary • The halogens all have seven electrons in their outer shells. • The electron configuration in the outer shell is $ns^2np^5$. • As the atomic number increases, the reactivity of the halogens decreases. • Fluorine and chlorine exist as gases at room temperature, while bromine is a liquid, and iodine is a solid. Review 1. Pick two elements that are halogens. For each, write the name, chemical symbol, and atomic weight. 2. What does the term “halogen” mean? 3. What is the outer shell electron configuration for the halogens? 4. What is the physical state of each halogen at room temperature? 5. Where are the halogens found? 6.13: Transition Elements What are the similarities and differences between these two cars? From the outside, the two cars below look the same (except for the flashy paint job on the racing model). They are the same model of the car, but one is a stock edition for regular driving, while the other one is built for high-speed racing. We really can't tell much from the external view. To see the differences, we need to go under the hood, take the engines apart, and look at the braking and suspension systems in order to see how the two cars differ. Many electron configurations of elements are simple and straightforward. We can look at the outer shell and easily understand how that set of elements will react in terms of electron gain or loss. However, there are sets of elements that are more complex in their behavior. One such group is the transition elements. The Transition Elements Transition elements are the elements that are found in Groups 3-12 (old groups IIA-IIB) on the periodic table (salmon-colored block in the middle of the table). The term refers to the fact that the $d$ sublevel, which is in the process of being filled, is in a lower principal energy level than the $s$ sublevel filled before it. For example, the electron configuration of scandium, the first transition element, is $\left[ \ce{Ar} \right] \: 3d^1 \: 4s^2$. Remember that the configuration is reversed from the fill order—the $4s$ fills before the $3d$ begins. Because they are all metals, the transition elements are often called the transition metals. As a group, they display typical metallic properties and are less reactive than the metals in Groups 1 and 2. Some of the more familiar ones are so unreactive that they can be found in nature in their free, or uncombined, state. These include platinum, gold, and silver. Because of this unique filling order, the transition elements are often referred to as "$d$-block" elements. Compounds of many transition elements are distinctive for being widely and vividly colored. As visible light passes through a transition metal compound dissolved in water, the $d$-orbitals absorb light of various energies. The visible light of a given energy level that is not absorbed produces a distinctly colored solution. Summary • The transition elements are found in groups IIA-IIB (new groups 3-12). • These elements are characterized by having unfilled $d$ sublevels. • In general, the next higher $s$ sublevel is already filled or has one electron missing. • Many transition element compounds are brightly colored due to the inner-level $d$ electron transitions. Review 1. List five different transition elements, giving their name, chemical symbol, and atomic number. 2. What is unique about the transition elements in terms of electron configurations? 3. Why are these elements often referred to as “$d$-block” elements? 4. Why do many transition element compounds have bright colors?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.12%3A_Halogens.txt
How many dolls are in this picture? Russian "nesting dolls" (often known as matryoshka dolls) have a long history in Russia. These dolls are designed to nest inside of one another. When we open the largest doll, we find a slightly smaller doll inside it. These dolls can often go down seven or eight layers, and some have over thirty-five layers. Lanthanides and Actinides We see some hidden "layers" in chemistry. Examining the periodic table below, visible are two pink boxes—one between $\ce{Ba}$ (element 56) and $\ce{Hf}$ (element 72) and the other between $\ce{Ra}$ (88) and $\ce{Rf}$ (104). These elements all have unfilled $f$-sublevels. Because of the uniqueness of the electron configurations, these elements fit into the two boxes in the larger periodic table. As the number of electrons in an atom increases, we begin to see some strange behaviors. Due to the way the electron energy levels work, some inner levels fill after one or more outer layers do. We see this in two similar groups of elements—the lanthanides and the actinides. The $f$-Block The first of the $f$ sublevels to begin filling is the $4f$ sublevel. It fills after the $6s$ sublevel, meaning that $f$ sublevels are two principal energy levels behind. The general electron configuration for elements in the $f$-block is $\left( n-2 \right) f^{1-14} ns^2$. The seven orbitals of the $f$ sublevel accommodate 14 electrons, so the $f$ block is 14 elements in length. It is pulled out of the main body of the periodic table and is shown at the very bottom. Because of that, the elements of the $f$ block do not belong to a group, being wedged in between Groups 3 and 4. The lanthanides are the 14 elements from cerium (atomic number 58) to lutetium (atomic number 71). The word comes from the Greek "$\lambda \alpha \nu \theta \alpha \nu \epsilon \iota \nu$" meaning "to be hidden". The name probably arose because these elements all hide behind one another in the periodic table. The $4f$ sublevel is in the process of being filled for the lanthanides. They are all metals and are similar in reactivity to the Group 2 alkaline earth metals. The actinides are the 14 elements from thorium (atomic number 90) to lawrencium (atomic number 103). The $5f$ sublevel is in the process of being filled. The actinides are all radioactive elements and only the first four have been found naturally on Earth. All of the others have only been artificially made in the laboratory. The lanthanides and actinides together are sometimes called the inner transition elements. Uses of Lanthanides Lanthanides have been widely used as alloys to impart strength and hardness to metals. The main lanthanide used for this purpose is cerium, mixed with small amounts of lanthanum, neodymium, and praseodymium. These metals are also widely used in the petroleum industry for refining crude oil into gasoline products. Erbium and other lanthanides are widely used in some optical devices, such as night vision goggles, laser beams, and phosphorescent materials. Uses of Actinides The actinides are valuable primarily because they are radioactive. These elements can be used as energy sources for applications as varied as cardiac pacemakers, to the generation of electrical energy for instruments on the moon. Uranium and plutonium have been employed in nuclear weapons and in nuclear power plants. Summary • Lanthanides and actinides are elements with unfilled $f$ orbitals. • Lanthanides are all metals with reactivity similar to group 2 elements. • Actinides are all radioactive elements. • Lanthanides are used in optical devices (night vision goggles), petroleum refining, and alloys. • Actinides are found primarily in applications where their radioactivity can be used to power devices such as cardiac pacemakers. Review 1. What electron sublevel is being filled in the lanthanides? 2. What electron sublevel is being filled in the actinides? 3. What sublevel is filled just prior to the filling of this sublevel? 4. Which actinides are found naturally on earth? 5. List some uses for lanthanides. 6. List some uses for actinides.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.14%3A_Lanthanides_and_Actinides.txt
How can all of these people fit in such a small space? Events draw large numbers of people to them. Even an outdoor event can fill up so that there is no room for more people. The crowd capacity depends on the amount of space in the venue, and the amount of space depends on the size of the objects filling it. We can get more people into a given space than can elephants, because elephants are larger than people. We can get more squirrels into that same space than we can people for the same reason. Knowing the sizes of objects to be dealt with can be important in deciding how much space is needed. Atomic Radius The size of atoms is important to explanations of the behavior of atoms or compounds. One way to express the size of atoms is by use of atomic radius. This data helps us understand why some molecules fit together and why other molecules have parts that get too crowded under certain conditions. The size of an atom is defined by the edge of its orbital. However, orbital boundaries are fuzzy, and variable under different conditions. In order to standardize the measurement of atomic radii, the distance between the nuclei of two identical atoms bonded together is measured. The atomic radius is defined as one-half the distance between the nuclei of identical atoms that are bonded together. Atomic radii have been measured for elements. The units for atomic radii are picometers, equal to $10^{-12}$ meters. As an example, the internuclear distance between the two hydrogen atoms in an $\ce{H_2}$ molecule is measured to be $74 \: \text{pm}$. Therefore, the atomic radius of a hydrogen atom is $\frac{74}{2} = 37 \: \text{pm}$. Periodic Trend The atomic radius of atoms generally decreases from left to right across a period. There are some small exceptions, such as the oxygen radius being slightly greater than the nitrogen radius. Within a period, protons are added to the nucleus as electrons are being added to the same principal energy level. These electrons are gradually pulled closer to the nucleus because of its increased positive charge. Since the force of attraction between nuclei and electrons increases, the size of the atoms decreases. The effect lessens as one moves further to the right in a period, because of electron-electron repulsions that would otherwise cause the atom's size to increase. Group Trend The atomic radius of atoms generally increases from top to bottom within a group. As the atomic number increases down a group, there is again an increase in the positive nuclear charge. However, there is also an increase in the number of occupied principal energy levels. Higher principal energy levels consist of orbitals which are larger in size than the orbitals from lower energy levels. The effect of the greater number of principal energy levels outweighs the increase in nuclear charge, and so atomic radius increases down a group. Summary • Atomic radius is determined as half the distance between the nuclei of two identical atoms bonded together. • The atomic radius of atoms generally decreases from left to right across a period. • The atomic radius of atoms generally increases from top to bottom within a group. Review 1. Define “atomic radius.” 2. What are the units of measurement for atomic radius? 3. How does the atomic radius of different elements change across a period? 4. How does atomic radius change from top to bottom within a group? 5. Explain why the atomic radius of hydrogen is so much smaller than the atomic radius of potassium.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.15%3A_Periodic_Trends-_Atomic_Radius.txt
The incredible green lights in this cold northern sky consist of charged particles known as ions. Their swirling pattern is caused by the pull of Earth’s magnetic field. Called the northern lights, this phenomenon of nature shows that ions respond to a magnetic field. Do you know what ions are? Read on to find out. Atoms Are Neutral The northern lights aren’t caused by atoms, because atoms are not charged particles. An atom always has the same number of electrons as protons. Electrons have an electric charge of -1 and protons have an electric charge of +1. Therefore, the charges of an atom’s electrons and protons “cancel out.” This explains why atoms are neutral in electric charge. Q: What would happen to an atom’s charge if it were to gain extra electrons? A: If an atom were to gain extra electrons, it would have more electrons than protons. This would give it a negative charge, so it would no longer be neutral. Atoms to Ions Atoms cannot only gain extra electrons. They can also lose electrons. In either case, they become ions. Ions are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions. Consider the example of fluorine (see figure below). A fluorine atom has nine protons and nine electrons, so it is electrically neutral. If a fluorine atom gains an electron, it becomes a fluoride ion with an electric charge of -1. Names and Symbols Like fluoride, other negative ions usually have names ending in –ide. Positive ions, on the other hand, are just given the element name followed by the word ion. For example, when a sodium atom loses an electron, it becomes a positive sodium ion. The charge of an ion is indicated by a plus (+) or minus sign (-), which is written to the right of and just above the ion’s chemical symbol. For example, the fluoride ion is represented by the symbol F-, and the sodium ion is represented by the symbol Na+. If the charge is greater than one, a number is used to indicate it. For example, iron (Fe) may lose two electrons to form an ion with a charge of plus two. This ion would be represented by the symbol Fe2+. This and some other common ions are listed with their symbols in the table below. Table $PageIndex{1}$: Some Common Ions Cations Anions Name of Ion Chemical Symbol Name of Ion Chemical Symbol Calcium ion Ca2+ Chloride Cl- Hydrogen ion H+ Fluoride F- Iron(II) ion Fe2+ Bromide Br- Iron(III) ion Fe3+ Oxide O2- Q: How does the iron(III) ion differ from the iron(II) ion? A: The iron(III) ion has a charge of +3, so it has one less electron than the iron(II) ion, which has a charge of +2. Q: What is the charge of an oxide ion? How does its number of electrons compare to its number of protons? A: An oxide ion has a charge of -2. It has two more electrons than protons. How Ions Form The process in which an atom becomes an ion is called ionization. It may occur when atoms are exposed to high levels of radiation. The radiation may give their outer electrons enough energy to escape from the attraction of the positive nucleus. However, most ions form when atoms transfer electrons to or from other atoms or molecules. For example, sodium atoms may transfer electrons to chlorine atoms. This forms positive sodium ions (Na+) and negative chloride ions (Cl-). Q: Why do you think atoms lose electrons to, or gain electrons from, other atoms? A: Atoms form ions by losing or gaining electrons because it makes them more stable and this state takes less energy to maintain. The most stable state for an atom is to have its outermost energy level filled with the maximum possible number of electrons. In the case of metals such as lithium, with just one electron in the outermost energy level, a more stable state can be achieved by losing that one outer electron. In the case of nonmetals such as fluorine, which has seven electrons in the outermost energy level, a more stable state can be achieved by gaining one electron and filling up the outer energy level. Properties of Ions Ions are highly reactive, especially as gases. They usually react with ions of opposite charge to form neutral compounds. For example, positive sodium ions and negative chloride ions react to form the neutral compound sodium chloride, commonly known as table salt. This occurs because oppositely charged ions attract each other. Ions with the same charge, on the other hand, repel each other. Ions are also deflected by a magnetic field, as you saw in the opening image of the northern lights. Summary • Atoms have equal numbers of positive protons and negative electrons, so they are neutral in electric charge. • Atoms can gain or lose electrons and become ions, which are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons. • The process in which an atom becomes an ion is called ionization. It may occur when atoms are exposed to high levels of radiation or when atoms transfer electrons to or from other atoms. • Ions are reactive, attracted or repulsed by other charged particles, and deflected by a magnetic field. Review 1. Why are atoms neutral in electric charge? 2. Define ion. 3. Compare and contrast cations and anions, and give an example of each. 4. Describe how ions form. 5. List properties of ions. 6. The model in the illustration below represents an atom of lithium (Li). If the lithium atom becomes an ion, which type of ion will it be, a cation or an anion? What will be the electric charge of this ion? What will the ion be named? What symbol will be used to represent it?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.16%3A_Ion.txt
Why do sheep travel in herds? Like many other animals, sheep travel in herds. The tendency is for each individual sheep to stay with the herd. However, a sheep may sometimes wander off, depending on how strong the attraction is for a particular food or water supply. At other times, a sheep may become frightened and run off. Whether a sheep chooses to stay with the herd or go its own way depends on the balance between attraction to the herd and attraction to the outside influence. There is an on-going tension between the electrons and protons in an atom. Reactivity of the atom depends in part on how easily the electrons can be removed from the atom. We can measure this quantity and use it to make predictions about the behaviors of atoms. Ionization Energy Ionization energy is the energy required to remove an electron from a specific atom. It is measured in kJ/mol, which is an energy unit, much like calories. The ionization energies associated with some elements are described in table below. For any given atom, the outermost valence electrons will have lower ionization energies than the inner-shell kernel electrons. As more electrons are added around a nucleus, the outer electrons become shielded from the nucleus by the inner shell electrons. This is called electron shielding. Table $PageIndex{1}$: Ionization Energies (kJ/mol) of the First 18 Elements Element IE1 IE2 IE3 IE4 IE5 IE6 H 1312 He 2373 5251 Li 520 7300 11,815 Be 899 1757 14,850 21,005 B 801 2430 3660 25,000 32,820 C 1086 2350 4620 6220 38,000 47,261 N 1400 2860 4580 7500 9400 53,000 O 1314 3390 5300 7470 11,000 13,000 If we plot the first ionization energies vs. atomic number for the main group elements, we would have the following trend Moving from left to right across the periodic table, the ionization energy for an atom increases. We can explain this by considering the nuclear charge of the atom. The more protons in the nucleus, the stronger the attraction of the nucleus to electrons. This stronger attraction makes it more difficult to remove electrons. Within a group, the ionization energy decreases as the size of the atom gets larger. On the graph, we see that the ionization energy increases as we go up the group to smaller atoms. In this situation, the first electron removed is farther from the nucleus as the atomic number (number of protons) increases. Being farther away from the positive attraction makes it easier for that electron to be pulled off. Summary • Ionization energy refers to the amount of energy needed to remove an electron from an atom. • Ionization energy decreases as we go down a group. • Ionization energy increases from left to right across the periodic table. Review 1. Define “ionization energy.” 2. Do valence electrons have larger or smaller ionization energies that the inner-shell kernel electrons? 3. What is electron shielding? 4. Describe the trends in ionization energy from left to right across the periodic table. 5. Describe the trends in ionization energy from top to bottom of a group in the periodic table. 6. Why is the second ionization energy for lithium so much larger than the first ionization energy?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.17%3A_Periodic_Trends_-_Ionization_Energy.txt
What is the goal of a roller derby game? Roller derby is a popular sport, although it is unfamiliar to many people. The basic purpose is to set one team member (the "jammer") past the opposing team to score points. Other members of the team serve as blockers to prevent the opposing team from stopping the jammer. Blockers interfere with the interaction between the jammer and the opponents by getting between the jammer and the skaters trying to stop them. The attraction between an electron and the nucleus of the atom is not a simple issue. Only with hydrogen is there a one-to-one relationship that can be discussed in terms of direct charge attraction. As the size of the atom increases, the number of protons and electrons also increase. These changes influence how the nucleus attracts electrons. Electron Shielding In general, the ionization energy of an atom will increase as we move from left to right across the periodic table. There are several exceptions to the general increase in ionization energy across a period. The elements of Group 13 ($\ce{B}$, $\ce{Al}$, etc.) have lower ionization energies than the elements of Group 2 ($\ce{Be}$, $\ce{Mg}$, etc.). This is an illustration of a concept called "electron shielding". Outer electrons are partially shielded from the attractive force of the protons in the nucleus by inner electrons. To explain how shielding works, consider a lithium atom. It has three protons and three electrons—two in the first principal energy level and its valence electron in the second. The valence electron is partially shielded from the attractive force of the nucleus by the two inner electrons. Removing that valence electron becomes easier because of the shielding effect. There is also a shielding effect that occurs between sublevels within the same principal energy level. Specifically, an electron in the $s$ sublevel is capable of shielding electrons in the $p$ sublevel of the same principal energy level. This is because of the spherical shape of the $s$ orbital. The reverse is not true—electrons in the $p$ orbitals do not shield electrons in $s$ orbitals. The electron being removed from an $\ce{Al}$ atom is a $3p$ electron, which is shielded by the two $3s$ electrons, as well as all the inner core electrons. The electron being removed from a $\ce{Mg}$ atom is a $3s$ electron, which is only shielded by the inner core electrons. Since there is a greater degree of electron shielding in the $\ce{Al}$ atom, it is slightly easier to remove the valence electron; its ionization energy is less than that of $\ce{Mg}$. This is despite the fact that the nucleus of the $\ce{Al}$ atom contains one more proton than the nucleus of the $\ce{Mg}$ atom. There is another anomaly between Groups 15 and 16. Atoms of Group 16 ($\ce{O}$, $\ce{S}$, etc.) have lower ionization energies than atoms of Group 15 ($\ce{N}$, $\ce{P}$, etc.). Hund's rule is behind the explanation. In a nitrogen atom, there are three electrons in the $2p$ sublevel and each is unpaired. In an oxygen atom, there are four electrons in the $2p$ sublevel, so one orbital contains a pair of electrons. It is that second electron in the orbital that is removed in the ionization of an oxygen atom. Since electrons repel each other, it is slightly easier to remove the electron from the paired set in the oxygen atom than it is to remove an unpaired electron from the nitrogen atom. Summary • Electron shielding refers to the blocking of valence shell electron attraction by the nucleus, due to the presence of inner-shell electrons. • Electrons in an $s$ orbital can shield $p$ electrons at the same energy level because of the spherical shape of the $s$ orbital. • Electrons in paired spin configurations are slightly easier to remove than unpaired electrons. Review 1. Define “electron shielding." 2. Why do group 13 elements have lower ionization energies than group 2 elements? 3. What influence does a larger shielding effect have on ionization energy? 4. How do s orbit electrons affect the ionization energy of a p electron in the same shell? 5. Why do group 16 atoms have lower ionization energies than the corresponding group 15 atoms?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.18%3A_Electron_Shielding.txt
Do you tend to overpack before going on trips? Packing for a trip can be very challenging. What do you take with you? Where will you be going and what will you need? We usually pack too much (like the suitcase above) and then find it hard to close the suitcase. When the suitcase is over-full, there is stress on the system and forces pushing the suitcase open. When electrons are added to an atom, the increased negative charge puts stress on the electrons already there, causing energy to be released. When electrons are removed from an atom, that process requires energy to pull the electron away from the nucleus. Addition of an electron releases energy from the process. Electron Affinity In most cases, the formation of an anion by the addition of an electron to a neutral atom releases energy. This can be shown for the chloride ion formation below: $\ce{Cl} + \ce{e^-} \rightarrow \ce{Cl^-} + \: \text{energy}\nonumber$ The energy change that occurs when a neutral atom gains an electron is called its electron affinity. When energy is released in a chemical reaction or process, that energy is expressed as a negative number. The figure below shows electron affinities in $\ce{kJ/mol}$ for the representative elements. Electron affinities are measured on atoms in the gaseous state and are very difficult to measure accurately. The elements of the halogen group (Group 17) gain electrons most readily, as can be seen from their large negative electron affinities. This means that more energy is released in the formation of a halide ion than for the anions of any other elements. Considering electron configuration, it is easy to see why. The outer configuration of all halogens is $ns^2 \: np^5$. The addition of one more electron gives the halide ions the same electron configuration as a noble gas, which we have seen is particularly stable. Period and group trends for electron affinities are not nearly as regular as for ionization energy. In general, electron affinities increase (become more negative) from left to right across a period and decrease (become less negative) from top to bottom down a group. However, there are many exceptions, owing in part to inherent difficulties in accurately measuring electron affinities. Summary • Electron affinity is a measure of the energy released when an extra electron is added to an atom. • Electron affinities are measured in the gaseous state. • In general, electron affinities become more negative as we move from left to right on the periodic table. • In general, electron affinities become less negative from top to bottom of a group. Review 1. Define “electron affinity." 2. Does addition of an electron to a neutral atom require energy to be absorbed or released? 3. Describe the general trend for electron affinity values moving from left to right on the periodic table. 4. Describe the general trend for electron affinity values moving from top to bottom in a group on the periodic table. 5. Why is more energy released in the formation of a halide ion than with other elements? 6.20: Periodic Trends - Ionic Radii How are peanuts sold? Peanuts can be sold two ways. The bare peanut without the shell (brown inner portion of peanut) can be purchased in jars and packages for casual munching or for cooking. The size of the peanut in this situation is smaller than the peanut plus shell since the outer portion is missing. If we add the shell to the peanut, we have a larger size for the combination. Electrons and protons are strongly attracted to one another. The strength of that attraction and the relative numbers of the two particles in a given atom or ion have a significant influence on the size of that species. When an atom loses one or more electrons, the resulting ion becomes smaller. If electrons are added to the atom, the ion becomes larger. Ionic Radius The ionic radius for an atom is measured in a crystal lattice, requiring a solid form for the compound. These radii will differ somewhat depending upon the technique used. Usually, x-ray crystallography is employed to determine the radius for an ion. The removal of electrons always results in a cation that is considerably smaller than the parent atom. When the valence electron(s) are removed, the resulting ion has one fewer occupied principal energy level, so the electron cloud that remains is smaller. Another reason is that the remaining electrons are drawn closer to the nucleus because the protons now outnumber the electrons. One other factor is the number of electrons removed. The potassium atom has one electron removed to form the corresponding ion, while calcium loses two electrons. The addition of electrons always results in an anion that is larger than the parent atom. When the electrons outnumber the protons, the overall attractive force that the protons have for the electrons is decreased. The electron cloud also spreads out because more electrons results in greater electron-electron repulsion. Notice that the group 16 ions are larger than the group 17 ions. The group 16 elements each add two electrons, while the group 17 elements add one electron per atom, to form the anions. Summary • Ionic radius is determined by measuring the atom in a crystal lattice. • Removal of electrons results in an ion that is smaller than the parent element. • Addition of electrons results in an ion that is larger than the parent atom. Review 1. How are ionic radii measured? 2. Explain why the radius of the rubidium ion is smaller than the radius of the rubidium atom. 3. Explain why the radius of the tellurium ion is larger than the radius of the tellurium atom. 4. Why is the oxygen anion larger than the fluoride anion? 5. Why is the sodium cation larger than the magnesium cation?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.19%3A_Periodic_Trends_-_Electron_Affinity.txt
Is it easy or hard for you to make new friends? Have you ever noticed how some people attract others to them? Whether it be their personality, attractiveness, or athletic skills—something pulls people toward them; while others have a smaller group of friends and acquaintances. Atoms do the same thing. One atom may pull electrons strongly to it, while a second type of atom has much less "pulling power". Electronegativity Valence electrons of both atoms are always involved when those two atoms come together to form a chemical bond. Chemical bonds are the basis for how elements combine with one another to form compounds. When these chemical bonds form, atoms of some elements have a greater ability to attract the valence electrons involved in the bond than other elements. Electronegativity is a measure of the ability of an atom to attract the electrons when the atom is part of a compound. Electronegativity differs from electron affinity because electron affinity is the actual energy released when an atom gains an electron. Electronegativity is not measured in energy units, but instead a relative scale. All elements are compared to one another, with the most electronegative element, fluorine, being assigned an electronegativity value of 3.98. Fluorine attracts electrons better than any other element. The table below shows the electronegativity values for the elements. Since metals have few valence electrons, they tend to increase their stability by losing electrons to become cations. Consequently, the electronegativities of metals are generally low. Nonmetals have more valence electrons and increase their stability by gaining electrons to become anions. The electronegativities of nonmetals are generally high. Trends Electronegativities generally increase from left to right across a period. This is due to an increase in nuclear charge. Alkali metals have the lowest electronegativities, while halogens have the highest. Because most noble gases do not form compounds, they do not have electronegativities. Note that there is little variation among the transition metals. Electronegativities generally decrease from top to bottom within a group, due to the larger atomic size. Of the main group elements, fluorine has the highest electronegativity (EN $= 4.0$) and cesium the lowest (EN $= 0.79$). This indicates that fluorine has a high tendency to gain electrons from other elements with lower electronegativities. We can use these values to predict what happens when certain elements combine. When the difference between atom electronegativities is greater than ~1.7, then a complete exchange of electrons occurs. Typically this exchange is between a metal and a nonmetal. For instance, sodium and chlorine will typically combine to form a new compound and each ion becomes isoelectronic with its nearest noble gas. When we compare the EN values, we see that the electronegativity for $\ce{Na}$ is 0.93 and the value for $\ce{Cl}$ is 3.2. The absolute difference between ENs is $\left\| 0.93 - 3.2 \right\| = 2.27$. This value is greater than 1.7, and therefore indicates the occurence of a complete electron exchange. Summary • Electronegativity is a measure of the ability of an atom to attract the electrons when the atom is part of a compound. • Electronegativity values generally increase from left to right across the periodic table. • Electronegativities generally decrease from the top to bottom of a group. • The highest electronegativity value is for fluorine. Review 1. Define “electronegativity.” 2. How does electronegativity differ from electron affinity? 3. Why are the electronegativity values of metals generally low? 4. Describe the trend in electronegativities across the periodic table. 5. Describe the trends in electronegativities in a group of the periodic table.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.21%3A_Periodic_Trends-_Electronegativity.txt
What are we eating as a nation? The graph above indicates some trends in the U.S. diet over a thirty-year period. By observing the direction our eating habits are going, steps can be taken to help prevent bad eating habits and decrease problems such as high blood pressure and heart attacks. Development of the periodic table has helped organize chemical information in many ways. We can now see trends among properties of different atoms and make predictions about the behavior of specific materials. Metallic and Nonmetallic Character Metallic character refers to the level of reactivity of a metal. Metals tend to lose electrons in chemical reactions, as indicated by their low ionization energies. Within a compound, metal atoms have relatively low attraction for electrons, as indicated by their low electronegativities. By following the trend summary in the figure below, you can see that the most reactive metals would reside in the lower left portion of the periodic table. The most reactive metal is cesium, which is not found in nature as a free element. It reacts explosively with water and will ignite spontaneously in air. Francium is below cesium in the alkali metal group, but is so rare that most of its properties have never been observed. Reactivity of metals is based on processes such as the formation of halide compounds with halogens, and how easily the element displaces hydrogen from dilute acids. The metallic character increases as you go down a group. Since the ionization energy decreases going down a group (or increases going up a group), the increased ability for metals lower in a group to lose electrons makes them more reactive. In addition, the atomic radius increases going down a group, placing the outer electrons further away from the nucleus and making that electron less attracted by the nucleus. Nonmetals tend to gain electrons in chemical reactions, and have a high attraction for electrons within a compound. The most reactive nonmetals reside in the upper right portion of the periodic table. Since the noble gases are a special group because of their lack of reactivity, the element fluorine is the most reactive nonmetal. It is not found in nature as a free element. Fluorine gas reacts explosively with many other elements and compounds, and is considered to be one of the most dangerous known substances. Note that there is no clear division between metallic and non-metallic character. As we move across the periodic table, there is an increasing tendency to accept electrons (nonmetallic) and a decrease in the possibility that an atom will give up one or more electrons. Summary • Metallic character refers to the level of reactivity of a metal. • Non-metallic character relates to the tendency to accept electrons during chemical reactions. • Metallic tendency increases going down a group. • Non-metallic tendency increases going from left to right across the periodic table. Review 1. Define “metallic character.” 2. Define “non-metallic character.” 3. Describe the trend in metallic character going down a group. 4. Describe the trend in non-metallic character going across the periodic table. 5. Why does the metallic character increase as you go down a group?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/06%3A_The_Periodic_Table/6.22%3A_Periodic_Trends_-_Metallic_and_Nonmetallic_Character.txt
The International Union of Pure and Applied Chemistry (IUPAC) is an international federation of organizations that represents chemists in individual countries. IUPAC is best known for its works standardizing nomenclature in chemistry and other fields of science. 07: Chemical Nomenclature Why are music notes a unique "language"? There are many "universal languages" in the world. Musicians of every culture recognize music embodied in a series of notes on a staff. This passage from a Bach cello suite could be played by any trained musician from any country, because there is an agreement as to what the symbols on the page mean. In the same way, molecules are represented using symbols that all chemists agree upon. Molecular Formula A molecule is comprised of two or more atoms that have been chemically combined. A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. Ammonia is a compound of nitrogen and hydrogen as shown below: Note from the example that there are some standard rules to follow in writing molecular formulas. The arrangements of the elements depend on the particular structure, so we will not concern ourselves with that point right now. The number of atoms of each kind is indicated by a subscript following the atom. If there is only one atom, no number is written. If there is more than one atom of a specific kind, the number is written as a subscript following the atom. We would not write $\ce{N_3H}$ for ammonia, because that would mean that there are three nitrogen atoms and one hydrogen atom in the molecule, which is incorrect. The molecular formula does not tell us anything about the shape of the molecule or where the different atoms are. The molecular formula for sucrose (table sugar) is $\ce{C_{12}H_{22}O_{11}}$. This simply tells us the number of carbon, hydrogen, and oxygen atoms in the molecule. There is nothing said about where the individual atoms are located. We need a much more complicated formula (shown below) to communicate that information. Summary • A molecular formula tells us which atoms and how many of each type of atom are present in a molecule. • If only one atom of a specific type is present, no subscript is used. • For atoms that have two or more of a specific type of atom present, a subscript is written after the symbol for that atom. • Molecular formulas do not indicate how the atoms are arranged in a molecule. Review 1. What does a molecular formula tell us? 2. What does a molecular formula not tell us? 3. What do the subscripts mean in a molecular formula? 4. If you wrote C6H11O5C6H11O6 as the molecular formula for sucrose, would that be correct? Explain your answer. 5. Sometimes the formula for acetic acid is written CH3COOH. Is this a true molecular formula? 7.02: Empirical Formula What might the scientists in the picture be discussing? When the French scientist Antoine Lavoisier conducted his experiments, he did not know what the products of each reaction would be. He had to isolate the material (whether he was heating mercury or measuring gases from breathing) and then study its elemental composition before he could understand the processes that were occurring. Discovering that a new compound exists is the start of a long research project. In order to make this new compound in the lab, we need to know a lot about its structure. Often, the place to start is to determine the elements in the material. Then, we can find out the relative amounts of each element to continue our evaluation of this new material. Empirical Formula An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular formula is $\ce{C_6H_{12}O_6}$. Since each of the subscripts is divisible by 6, the empirical formula for glucose is $\ce{CH_2O}$. When chemists analyze an unknown compound, often the first step is to determine its empirical formula. There are a great many compounds whose molecular and empirical formulas are the same. If the molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of $\ce{H_2O}$ or $\ce{P_2O_5}$, then the empirical formula is also the molecular formula. How do we determine an empirical formula for a compound? Consider a compound composed of carbon, hydrogen, and oxygen. We can analyze the relative amounts of each element in the compound. When we get a percent figure for each element, we now know how many grams of each are in 100 grams of the original material. This allows us to determine the number of moles for each element. The ratios can then be reduced to small whole numbers to give the empirical formula. If we wanted a molecular formula, we would need to determine the molecular weight of the compound. Summary • The empirical formula gives the lowest whole-number ratio of elements in a compound. • The empirical formula does not show the actual number of atoms. Review 1. Define “empirical formula.” 2. Why is C6H12O6 not considered to be an empirical formula for glucose? 3. Can the empirical formula for a compound be the same as the molecular formula? 4. What do we need to know in order to determine a molecular formula from an empirical formula? 5. Give three examples of compounds whose empirical formulas are the same as their molecular formulas.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.01%3A_Molecular_Formula.txt
Have you ever gone digging for gold? When the prospectors during the California Gold Rush (1848-1855) searched for gold nuggets in the earth, they were able to find the nuggets because gold is an unreactive material that exists in its elemental state in many places. Not everyone was fortunate enough to find significantly large gold nuggets, but a number of miners did become very wealthy (of course, a large number of others went back home broke). Many of the elements we know about do not exist in their native form. They are so reactive that they are found only in compounds. These non-elemental forms are known as ions. Their properties are very different from those of the elements they come from. The term comes from a Greek word meaning "move" and was first coined by Michael Faraday, who studied the movement of materials in an electrical field. Cations Some elements lose one or more electrons in forming ions. These ions are known as "cations" because they are positively charged and migrate toward the negative electrode (cathode) in an electrical field. Looking at the periodic table below, we know that the group 1 elements are all characterized by having one $s$ electron in the outer orbit; group 2 elements have two $s$ electrons in the outer orbit. These electrons are loosely attached to the atom and can easily be removed, leaving more protons in the atom than there are electrons, so that the resulting ion has a positive charge. Cations can also be formed from electron loss to many of the transition elements. The cations are designated by the symbol for the parent element and a plus charge as a superscript after the element symbol—the potassium cation would be indicated as $\ce{K^+}$. Note that the charge is placed after the symbol and not before it. The potassium ion is monovalent, meaning that it has lost one electron and has a $+1$ charge. The symbol for the magnesium cation would be $\ce{Mg^{2+}}$ or $\ce{Mg^{++}}$ to indicate that it has lost two electrons and has a $+2$ charge, so the magnesium cation would be referred to as a divalent cation. The cations are simply named as the parent element. The sodium cation is still called "sodium". Often, the charge would be attached for clarity, so the sodium cation might be referred to as "sodium one plus". Applications of Cations Cations play important roles in our daily lives. Sodium, potassium, and magnesium ions are essential for such processes as blood pressure regulation and muscle contraction. Calcium ions are an important part of bone structure. Sodium ions can be used in water softeners to remove other harmful elements. We put sodium chloride (table salt) on our food and use it as a preservative. Summary • Cations are formed by the loss of one or two electrons from an element. • Groups 1 and 2 elements form cations. • Cations are named according to the parent element. • Cation charges are indicated with a superscript following the chemical symbol. Review 1. What is a cation? 2. Write the symbol for the barium cation. 3. Write the symbol for the cesium cation. 4. List three ways cations are useful. 7.04: Anions What does the amount of salt in seaweed tell us? Before iodized salt was developed, some people experienced a number of developmental difficulties, including problems with thyroid gland function and intellectual disabilities. In the 1920s, we learned that these conditions could usually be treated easily with the addition of iodide anion to the diet. One easy way to increase iodide intake was to add the anion to table salt. This simple step greatly enhanced health and development. Large amounts of iodide ion are also found in seaweed such as kelp (see picture above) and saltwater fish. When a metal loses an electron, energy is needed to remove that electron. The other part of this process involves the addition of the electron to another element. The electron adds to the outer shell of the new element. Just as the loss of the electron from the metal produces a full shell, when the electron or electrons are added to the new element, it also results in a full shell. Anions Anions are negative ions that are formed when a nonmetal atom gains one or more electrons. Anions are so named because they are attracted to the anode (positive field) in an electrical field. Atoms typically gain electrons so that they will have the electron configuration of a noble gas. All the elements in Group 17 have seven valence electrons due to the outer $ns^2 \: np^5$ configuration. Therefore, each of these elements would gain one electron and become an anion with a $-1$ charge. Likewise, Group 16 elements form ions with a $-2$ charge, and the Group 15 nonmetals form ions with a $-3$ charge. Naming anions is slightly different than naming cations. The ending of the element's name is dropped and replaced with the -ide suffix. For example, $\ce{F^-}$ is the fluoride ion, while $\ce{O^{2-}}$ is the oxide ion. As is the case with cations, the charge on the anion is indicated by a superscript following the symbol. Common anions are listed in the table below: Anion Name Symbol and Charge Table $1$ fluoride $\ce{F^-}$ chloride $\ce{Cl^-}$ bromide $\ce{Br^-}$ iodide $\ce{I^-}$ oxide $\ce{O^{2-}}$ sulfide $\ce{S^{2-}}$ nitride $\ce{N^{3-}}$ Uses for Anions Fluoride ion is widely used in water supplies to help prevent tooth decay. Chloride is an important component in ion balance in blood. Iodide ion is needed by the thyroid gland to make the hormone thyroxine. Summary • Anions are formed by the addition of one or more electrons to the outer shell of an atom. • Group 17 elements add one electron to the outer shell, group 16 elements add two electrons, and group 15 elements add three electrons. • Anions are named by dropping the ending of the element's name and adding -ide. Review 1. What is an anion? 2. How are anions formed? 3. Why do anions form? 4. How are anions named? 5. List three examples of anions with names, charges, and chemical symbols. 6. List three ways anions are used.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.03%3A_Cations.txt
What kind of coin is this? Most of us are familiar with the common coins: penny, nickel, dime, quarter. In some areas (such as Las Vegas), you might see large amounts of silver dollars (these get a little heavy in your pocket). But most of us have probably never seen a platinum eagle—an eagle-depicting coin that is held primarily by collectors. If you were to take a one-ounce platinum eagle into a store to buy one hundred dollars worth of items, the store owner will most likely not believe you when you claim that the coin is worth one hundred dollars. It would also be awkward and annoying if you lost one of these coins out of your pocket. Platinum is just one of several transition metals that is worth a lot of money (gold is another one). The group 1 and 2 elements form cations through a simple process that involves the loss of one or more outer shell electrons. These electrons come from the $s$ orbital and are removed very readily. Transition Metal Ions Most transition metals differ from the metals of Groups 1, 2, and 13 in that they are capable of forming more than one cation with different ionic charges. As an example, iron commonly forms two different ions. It can sometimes lose two electrons to form the $\ce{Fe^{2+}}$ ion, while at other times it loses three electrons to form the $\ce{Fe^{3+}}$ ion. Tin and lead, though members of the $p$ block rather than the $d$ block, also are capable of forming multiple ions. Ionic formation for transition metals is complicated by the fact that these elements have unfilled inner $d$ shells. Although the next higher $s$ orbitals are actually at a lower energy level than the $d$ level, these $s$ electrons are the ones that are removed during ionization. The table below lists the names and formulas of some of the common transition metal ions: Common Transition Metal Ions Table $1$: Common Transition Metal Ions $1+$ $2+$ $3+$ $4+$ copper (I), $\ce{Cu^+}$ cadmium, $\ce{Cd^{2+}}$ chromium (III), $\ce{Cr^{3+}}$ lead (IV), $\ce{Pb^{4+}}$ gold (I), $\ce{Au^+}$ chromium (II), $\ce{Cr^{2+}}$ cobalt (III), $\ce{Co^{3+}}$ tin (IV), $\ce{Sn^{4+}}$ mercury (I), $\ce{Hg_2^{2+}}$ cobalt (II), $\ce{Co^{2+}}$ gold (III), $\ce{Au^{3+}}$ silver, $\ce{Ag^+}$ copper (II), $\ce{Cu^{2+}}$ iron (III), $\ce{Fe^{3+}}$ iron (II), $\ce{Fe^{2+}}$ lead (II), $\ce{Pb^{2+}}$ manganese (II), $\ce{Mn^{2+}}$ mercury (II), $\ce{Hg^{2+}}$ nickel (II), $\ce{Ni^{2+}}$ platinum (II), $\ce{Pt^{2+}}$ tin (II), $\ce{Sn^{2+}}$ zinc, $\ce{Zn^{2+}}$ Uses for Transition Metals Because there are so many metals in this group, there are a wide variety of uses. Many of the metals are used in electronics, while others (such as gold and silver) are used in monetary systems. Iron is a versatile structural material. Cobalt, nickel, platinum, and other metals are employed as catalysts in a number of chemical reactions. Zinc is a significant component of batteries. Summary • Transition metals have unfilled inner $d$ electron shells. • Ions form primarily through loss of $s$ electrons. • Many transition metals can form more than one ion. • Transition metals have a wide variety of applications. Review 1. What is unique about the electron configurations of transition metals? 2. Which electrons of transition metal elements are most likely to be lost during ion formation? 3. How many ions can iron form? 4. Which transition metal forms only one ion? 5. List several uses for transition metals.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.05%3A_Transition_Metal_Ions.txt
How is an mp3 player designed? In describing many technological items, it's not enough to simply say what brand or model we have. We talk about details such as how much horsepower is “under the hood” for a car or how fast the chip is for our computer. Even a simple device like an mp3 player has more than one size. We can get an 8 MB player, or a 16 MB player. Designation of the item often is incomplete without other information as to its capabilities. Transition metals have more than one possibility for ion formation. In order to name these compounds correctly, we need to be able to indicate which ion is involved in any given compound. Naming Compounds Using the Stock System Naming compounds that involve transition metal cations necessitates use of the Stock system. Consider the binary ionic compound $\ce{FeCl_3}$. To simply name this compound "iron chloride" would be incomplete, because iron is capable of forming two ions with different charges. The name of any iron-containing compound must reflect which iron ion is in the compound. In this case, the subscript in the formula indicates that there are three chloride ions, each with a $1-$ charge. Therefore, the charge of the single iron ion must be $3+$. The correct name of $\ce{FeCl_3}$ is iron (III) chloride, with the cation charge written as the Roman numeral. Here are several other examples: Formula Name Table $1$ $\ce{Cu_2O}$ copper (I) oxide $\ce{CuO}$ copper (II) oxide $\ce{SnO_2}$ tin (IV) oxide The first two examples are both oxides of copper (shown in the figure below). The ratio of copper ions to oxide ions determines the name. Since the oxide ion is $\ce{O^{2-}}$, the charges of the copper ion must be $1+$ in the first formula and $2+$ in the second formula. In the third formula, there is one tin ion for every two oxide ions. This means that the tin must carry a $4+$ charge, making the name tin (IV) oxide. Summary • The Stock system allows for the specification of transition metal ionic charge when naming ionic compounds. • Roman numerals are used to indicate the amount of positive charge on the cation. Review 1. What is the Stock system? 2. For which group of metal ions would we use the Stock system? 3. What does the Roman numeral stand for? 4. Assign a Roman numeral to each of the following cations: 1. Sn4+ 2. Fe3+ 3. Co2+ 4. Pb4+ 7.07: Naming Binary Ionic Compounds Do you know the proper name for this insect? Proper naming (or nomenclature) is important for identification purposes. Medicine names must be precise so that the correct drug is given—one that will help the patient and not harm them. Biological classification of species requires accurate naming for proper categorization. Insects are categorized by genus and species, so they are uniquely identified. Names must be correct for the tracing of family trees in genealogical studies. Compounds used in chemical reactions must be correctly specified in order for the reaction to occur. Binary Ionic Compounds A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. Naming Binary Ionic Compounds When examining the formula of a compound in order to name it, you must first decide what kind of compound it is. For a binary ionic compound, a metal will always be the first element in the formula, while a nonmetal will always be the second. The metal cation is named first, followed by the nonmetal anion. Subscripts in the formula do not affect the name. The table below shows three examples. Formula Name Table $1$: Naming Binary Ionic Compounds $\ce{KF}$ potassium fluoride $\ce{Na_3N}$ sodium nitride $\ce{Ca_3P_2}$ calcium phosphide Notice that in each of the formulas above, the overall charge of the compound is zero. Potassium ion is $\ce{K^+}$, while fluoride ion is $\ce{F^-}$. Since the magnitude of the charges is equal, the formula contains one of each ion. This would also be the case for a compound such as $\ce{MgS}$, in which the ions are $\ce{Mg^{2+}}$ and $\ce{S^{2-}}$. For sodium nitride, the sodium ion is $\ce{Na^+}$, while the nitride ion is $\ce{N^{3-}}$. In order to make a neutral compound, three of the $1+$ sodium ions are required in order to balance out the single $3-$ nitride ion. So the $\ce{Na}$ is given a subscript of 3. For calcium phosphide, the calcium ion is $\ce{Ca^{2+}}$, while the phosphide ion is $\ce{P^{3-}}$. The least common multiple of 2 and 3 is 6. To make the compound neutral, three calcium ions have a total charge of $6+$, while two phosphide ions have a total charge of $6-$. The $\ce{Ca}$ is given a subscript of 3, while the $\ce{P}$ is given a subscript of 2. Summary • A binary ionic compound is a compound composed of a monatomic metal cation and a monatomic nonmetal anion. • The metal part of the compound is named as the element. • The nonmetallic part of the compound is named by dropping the end of the element and adding -ide. • For binary compounds, it is not necessary to indicate the number of ions in the compound. Review 1. What is a binary compound? 2. Which of the following is not a binary compound? 1. NaCl 2. KH2PO4 3. KBr 3. Name the following compounds: 1. NaBr 2. MgCl2 3. LiI 4. CaO 5. CuBr2 6. FeO	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.06%3A_The_Stock_System_of_Nomenclature.txt
How does shorthand work? Shorthand was a very popular way of recording speech, especially in dictating letters and in court testimony. Instead of trying to write out all the words, the person taking the dictation would use a set of symbols that represented syllables or words. The pages above show a shorthand version of “A Christmas Carol” written by Charles Dickens. Unless you know shorthand, the passage is meaningless. But knowing shorthand allows you to read this classic story. Different professions also use a type of shorthand in communication to save time. Chemists use chemical symbols in combination to indicate specific compounds. There are two advantages to this approach: 1. The compound under discussion is clearly described so that there can be no confusion about its identity. 2. Chemical symbols represent a universal language that all chemists can understand, no matter what their native language is. Writing Formulas for Binary Ionic Compounds If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charge to cancel each other out. Consider the compound aluminum nitride. The ions are: $\ce{Al^{3+}} \: \: \: \: \: \ce{N^{3-}}\nonumber$ Since the ions have charges that are equal in magnitude, one of each will be the lowest ratio of ions in the formula. The formula for aluminum nitride is $\ce{AlN}$. The ions for the compound lithium oxide are: $\ce{Li^+} \: \: \: \: \: \ce{O^{2-}}\nonumber$ In this case, two lithium ions are required to balance out the charge of one oxide ion. The formula of lithium oxide is $\ce{Li_2O}$. An alternative way to write a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped. Shown below is the crisscross method for aluminum oxide. The red arrows indicate that the 3 from the $3+$ charge will cross over to become the subscript of the $\ce{O}$. The 2 from the $2-$ charge will cross over to become the subscript of the $\ce{Al}$. The formula for aluminum oxide is $\ce{Al_2O_3}$. Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions. In the case of aluminum nitride, the crisscross method would yield a formula of $\ce{Al_3N_3}$, which is not correct. It must be reduced to $\ce{AlN}$. Following the crisscross method to write the formula for lead (IV) oxide would involve the following steps: The crisscross first yields $\ce{Pb_2O_4}$ for the formula, but that must be reduced to the lower ratio, and $\ce{PbO_2}$ is the correct formula. Summary • Formulas for binary compounds begin with the metal followed by the nonmetal. • Positive and negative charges must cancel each other out. • Ionic compound formulas are written using the lowest ratio of ions. Review 1. Write formulas for the binary ionic compounds formed between the following pairs of elements: 1. cesium and fluorine 2. calcium and sulfur 3. aluminum and chlorine 4. zinc and nitrogen 2. Write the formula and give the name for the compound formed by the following ions: 1. Fe3+ and O2- 2. Ni2+ and S2- 3. Au+ and Cl- 4. Sn4+ and I- 3. Give names for the following compounds: 1. Ag2S 2. PdO 3. PtCl4 4. V2O5	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.08%3A_Formulas_for_Binary_Ionic_Compounds.txt
Have you ever read the story of Romeo and Juliet? Shakespeare wrote, "A rose by any other name would smell as sweet" as part of the romantic balcony scene between Romeo and Juliet in the famous play. The two families were bitter rivals, but Juliet meant this statement to mean that she loved Romeo no matter what his name was. Some names are simple—we know Romeo mainly as Romeo. Most of us have a first name, middle name (often not used), and last name. In some cultures, names will be much more complex. The full name of the famous 20$^\text{th}$ century artist Pablo Picasso is Pablo Diego José Francisco de Paula Juan Neponuceno María de los Remedios Cipriano de la Santísima Trinidad Martyr Patricio Clito Ruíz y Picasso. Many materials exist as simply binary compounds composed of a metal cation and a nonmetal anion, with each ion consisting of only one type of atom. Other combinations of atoms also exist, either as larger ionic complexes or complete molecules. Some of the most useful materials we work with contain polyatomic ions. Polyatomic Ions A polyatomic ion is an ion composed of more than one atom. The ammonium ion consists of one nitrogen atom and four hydrogen atoms. Together, they comprise a single ion with a $1+$ charge and a formula of $\ce{NH_4^+}$. The carbonate ion consists of one carbon atom and three oxygen atoms, and carries an overall charge of $2-$. The formula of the carbonate ion is $\ce{CO_3^{2-}}$. The atoms of a polyatomic ion are tightly bonded together, and so the entire ion behaves as a single unit. The figures below show several models. The table below lists a number of polyatomic ions by name and by formula. The heading for each column indicates the charge on the polyatomic ions in that group. Note that the vast majority of the ions listed are anions—there are very few polyatomic cations. $1-$ $2-$ $3-$ $1+$ $2+$ Table $1$: Common Polyatomic Ions acetate, $\ce{CH_3COO^-}$ carbonate, $\ce{CO_3^{2-}}$ arsenate, $\ce{AsO_3^{3-}}$ ammonium, $\ce{NH_4^+}$ dimercury, $\ce{Hg_2^{2+}}$ bromate, $\ce{BrO_3^-}$ chromate, $\ce{CrO_4^{2-}}$ phosphite, $\ce{PO_3^{3-}}$ chlorate, $\ce{ClO_3^-}$ dichromate, $\ce{Cr_2O_7^{2-}}$ phosphate, $\ce{PO_4^{3-}}$ chlorite, $\ce{ClO_2^-}$ hydrogen phosphate, $\ce{HPO_4^{2-}}$ cyanide, $\ce{CN^-}$ oxalate, $\ce{C_2O_4^{2-}}$ dihydrogen phosphate, $\ce{H_2PO_4^-}$ peroxide, $\ce{O_2^{2-}}$ hydrogen carbonate, $\ce{HCO_3^-}$ silicate, $\ce{SiO_3^{2-}}$ hydrogen sulfate, $\ce{HSO_4^-}$ sulfate, $\ce{SO_4^{2-}}$ hydrogen sulfide, $\ce{HS^-}$ sulfite, $\ce{SO_3^{2-}}$ hydroxide, $\ce{OH^-}$ hypochlorite, $\ce{ClO^-}$ nitrate, $\ce{NO_3^-}$ nitrite, $\ce{NO_2^-}$ perchlorate, $\ce{ClO_4^-}$ permanganate, $\ce{MnO_4^-}$ The vast majority of polyatomic ions are anions, many of which end in -ate or -ite. Notice that in some cases, such as nitrate $\left( \ce{NO_3^-} \right)$ and nitrite $\left( \ce{NO_2^-} \right)$, there are multiple anions that consist of the same two elements. In these cases, the difference between the ions is in the number of oxygen atoms present, while the overall charge is the same. As a class, these are called oxoanions. When there are two oxoanions for a particular element, the one with the greater number of oxygen atoms gets the -ate suffix, while the one with the fewer number of oxygen atoms gets the -ite suffix. The four oxoanions of chlorine are as follows: • $\ce{ClO^-}$, hypochlorite • $\ce{ClO_2^-}$, chlorite • $\ce{ClO_3^-}$, chlorate • $\ce{ClO_4^-}$, perchlorate In cases such as this, the ion with one more oxygen atom than the -ate anion is given a per- prefix. The ion with one fewer oxygen atom than the -ite anion is given a hypo- prefix. Summary • Polyatomic ions contain more than one type of atom in the ion. • The majority of polyatomic ions are anions that are named ending in -ate or -ite. • Some anions have multiple forms and are named accordingly. Review 1. Write the formulas for the following ions: 1. ammonium 2. carbonate 3. sulfate 4. phosphate 2. Name the following ions: 1. $\ce{PO_3^{3-}}$ 2. $\ce{SiO_3^{2-}}$ 3. $\ce{OH^-}$ 4. $\ce{MnO_4^-}$	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.09%3A_Polyatomic_Ions.txt
How many varieties of pizza can you think of? Many people enjoy eating pizza. When someone orders a pizza (either by phone, online, or in a restaurant), they don't just order "pizza". Even the simplest "binary" pizza will have a topping and cheese—maybe pepperoni, maybe something else. However, many pizzas quickly become more complex. One person may want pepperoni and sausage, another may wish to order Canadian bacon and pineapple; then you have the folks that can't decide, so they order half-this and half-that. The combinations may be more complex, but the same basic ideas about pizza are valid. Ternary Ionic Compounds Not all ionic compounds are composed of only monatomic ions. A ternary ionic compound is an ionic compound composed of three elements. In a typical ternary ionic compound, there is still one type of cation and one type of anion involved. The cation, the anion, or both, are polyatomic ions. Naming Ternary Ionic Compounds The process of naming ternary ionic compounds is the same as naming binary ionic compounds. The cation is named first, followed by the anion. Some examples are shown in the table below: Formula Name Table $1$: Examples of Ternary Ionic Compounds $\ce{NaNO_3}$ sodium nitrate $\ce{NH_4Cl}$ ammonium chloride $\ce{Fe(OH)_3}$ iron (III) hydroxide When more than one polyatomic ion is present in a compound, the formula of the ion is placed in parentheses with a subscript outside of the parentheses that indicates how many of those ions are in the compound. In the last example above, there is one $\ce{Fe^{3+}}$ cation and three $\ce{OH^-}$ anions. Writing Formulas for Ternary Ionic Compounds Writing a formula for a ternary ionic compound also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion. Use the crisscross method to ensure that the final formula is neutral. Calcium nitrate is composed of a calcium cation and a nitrate anion. The charge is balanced by the presence of two nitrate ions and one calcium ion. Parentheses are used around the nitrate ion because more than one of the polyatomic ion is needed. If only one polyatomic ion is in a formula, parentheses are not used. As an example, the formula for calcium carbonate is $\ce{CaCO_3}$. The carbonate ion carries a $2-$ charge and so exactly balances the $2+$ charge of the calcium ion. There are two polyatomic ions that produce unusual formulas. The $\ce{Hg_2^{2+}}$ ion is called either the dimercury ion or, preferably, the mercury (I) ion. When bonded with an anion with a $1-$ charge, such as chloride, the formula is $\ce{Hg_2Cl_2}$. Because the cation consists of two $\ce{Hg}$ atoms bonded together, this formula is not reduced to $\ce{HgCl}$. Likewise, the peroxide ion, $\ce{O_2^{2-}}$, is also a unit that must stay together in its formulas. For example, the formula for potassium peroxide is $\ce{K_2O_2}$. Summary • Ternary compounds are composed of three or more elements. • Ternary compounds are named by stating the cation first, followed by the anion. • Positive and negative charges must balance. Review 1. What is a ternary compound? 2. What is the basic rule for naming ternary compounds? 3. Write the formulas for the following compounds: 1. mercury(II) nitrate 2. ammonium phosphate 3. calcium silicate 4. lead(II) chromate 4. Name the following compounds: 1. KClO3 2. Rb2SO4 3. Cd(NO3)2 4. NaCN	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.10%3A_Ternary_Ionic_Compounds%3A_Naming_and_Formulas.txt
Why do so many relatives in royalty share the same name? Some families name a son (usually the first born) after his father. So, it is somewhat common to meet a John Smith, Jr. named after John Smith the father. Certain families may name a grandson John Smith III. Countries with long histories of royalty take the naming even further. One line of kings named Henry goes up to Henry the Eighth (not the nicest guy in the world—he had six wives and two of them met untimely ends). The use of numbering for names adds clarity to a system—it's easily discernible which Henry is being spoken of. Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular compounds. The structure of all ionic compounds is an extended three-dimensional array of alternating positive and negative ions. Since ionic compounds do not take the form of individual molecules, they are represented by empirical formulas. Now we will begin to examine the formulas and nomenclature of molecular compounds. Molecular Compounds Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substances as water $\left( \ce{H_2O} \right)$ and carbon dioxide $\left( \ce{CO_2} \right)$. These compounds are very different from ionic compounds like sodium chloride $\left( \ce{NaCl} \right)$. Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other. So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their valence electrons in such a way that a bond forms between pairs of atoms. In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms. Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample of a given molecular compound, all of the individual molecules are identical. Naming Binary Molecular Compounds Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is $\ce{H_2O}$. A molecule of octane, which is a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is $\ce{C_8H_{18}}$. A binary molecular compound is a molecular compound that is composed of two elements. The elements that combine to form binary molecular compounds are both nonmetal atoms. This contrasts with ionic compounds, which are formed from a metal ion and a nonmetal ion. Therefore, binary molecular compounds are different because ionic charges cannot be used to name them or to write their formulas. Another difference is that two nonmetal atoms will frequently combine with one another in a variety of ratios. Consider the elements nitrogen and oxygen. They combine to make several compounds including $\ce{NO}$, $\ce{NO_2}$, and $\ce{N_2O}$. They cannot all be called nitrogen oxide. How would someone know which one you were talking about? Each of the three compounds has very different properties and reactivities. A system to distinguish between compounds such as these is necessary. Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element. The table below shows the prefixes up to ten. Number of Atoms Prefix Table $1$: Numerical Prefixes 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- 9 nona- 10 deca- The rules for using the prefix system of nomenclature of binary compounds can be summarized as follows: 1. Generally, the less electronegative element is written first in the formula, though there are a few exceptions. Carbon is always first in a formula and hydrogen is after nitrogen in a formula such as $\ce{NH_3}$. The order of common nonmetals in binary compound formulas is $\ce{C}$, $\ce{P}$, $\ce{N}$, $\ce{H}$, $\ce{S}$, $\ce{I}$, $\ce{Br}$, $\ce{Cl}$, $\ce{O}$, $\ce{F}$. 2. When naming, the appropriate prefix is used only if there are more than one atom of that element in the formula. 3. The second element is named after the first, but with the ending of the element's name changed to -ide. The appropriate prefix is always used for the second element. 4. The a or o at the end of a prefix is usually dropped from the name when the name of the element begins with a vowel. As an example, four oxygen atoms are tetroxide instead of tetraoxide. Some examples of molecular compounds are listed in Table $2$. Formula Name Table $2$ $\ce{NO}$ nitrogen monoxide $\ce{N_2O}$ dinitrogen monoxide $\ce{S_2Cl_2}$ disulfur dichloride $\ce{Cl_2O_7}$ dichlorine heptoxide Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both of the first two examples. The $\ce{S_2Cl_2}$ emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios. The o of the mono- and the a of hepta- are dropped from the name when paired with oxide. Summary • Molecular compounds are inorganic compounds that take the form of discrete molecules. • The atoms of these compounds are held together by covalent bonds. • Prefixes are used to indicate the number of atoms of an element that are in the compound. Review 1. What are molecular compounds? 2. How do they differ from ionic compounds? 3. What holds the atoms of a molecule together? 4. Are the elements of molecular compounds metals or non-metals? 5. What is the prefix for the number of atoms of an element when there are four atoms of that element in the compound? 6. What is the prefix for the number of atoms of an element when there are 7 atoms of that element in the compound? 7. Name the following compounds: 1. ClF3 2. As2O5 3. B4H10	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.11%3A_Binary_Molecular_Compounds%3A_Naming_and_Formulas.txt
How is gold tested? A spot test for gold has been in use for decades. The sample is first treated with nitric acid. Other metals may react or dissolve in this acid, but gold will not. Then the sample is added to a mixture of nitric acid and hydrochloric acid. Gold will only dissolve in this mixture. The term "acid test" arose from the California gold rush in the late 1840s, when this combination was used to test for the presence of real gold. It has since come to mean "tested and approved" in a number of fields. Acids An acid can be defined in several ways. The most straightforward definition is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions $\left( \ce{H^+} \right)$ when dissolved in water. This is a different type of compound than the others we have seen so far. Acids are molecular, which means that in their pure state they are individual molecules and do not adopt the extended three-dimensional structures of ionic compounds like $\ce{NaCl}$. However, when these molecules are dissolved in water, the chemical bond between the hydrogen atom and the rest of the molecule breaks, leaving a positively-charged hydrogen ion and an anion. This can be symbolized in a chemical equation: $\ce{HCl} \rightarrow \ce{H^+} + \ce{Cl^-}\nonumber$ Since acids produce $\ce{H^+}$ cations upon dissolving in water, the $\ce{H}$ of an acid is written first in the formula of an inorganic acid. The remainder of the acid (other than the $\ce{H}$) is the anion after the acid dissolves. Organic acids are also an important class of compounds, but will not be discussed here. A binary acid is an acid that consists of hydrogen and one other element. The most common binary acids contain a halogen. An oxoacid is an acid that consists of hydrogen, oxygen, and a third element. The third element is usually a nonmetal. Naming Acids Since all acids contain hydrogen, the name of an acid is based on the anion that goes with it. These anions can either be monatomic or polyatomic. The name of all monatomic ions ends in -ide. The majority of polyatomic ions end in either -ate or -ite, though there are a few exceptions such as the cyanide ion $\left( \ce{CN^-} \right)$. It is the suffix of the anion that determines how the acid is named, as displayed in the rules and table below. Anion Suffix Example Name of Acid Example Table $1$: Naming System for Acids -ide chloride $\left( \ce{Cl^-} \right)$ hydro_____ic acid hydrochloric acid $\left( \ce{HCl} \right)$ -ate sulfate $\left( \ce{SO_4^{2-}} \right)$ ______ic acid sulfuric acid $\left( \ce{H_2SO_4} \right)$ -ite nitrite $\left( \ce{NO_2^-} \right)$ ____ous acid nitrous acid $\left( \ce{HNO_2} \right)$ The three different suffixes that are possible for the anions lead to three rules: 1. When the anion ends in -ide, the acid name begins with the prefix hydro-. The root of the anion name goes in the blank (chlor for chloride), followed by the suffix -ic. $\ce{HCl}$ is hydrochloric acid because $\ce{Cl^-}$ is the chloride ion. $\ce{HCN}$ is hydrocyanic acid because $\ce{CN^-}$ is the cyanide ion. 2. When the anion ends in -ate, the name of the acid is the root of the anion followed by the suffix -ic. There is no prefix. $\ce{H_2SO_4}$ is sulfuric acid (not sulfic) because $\ce{SO_4^{2-}}$ is the sulfate ion. 3. When the anion ends in -ite, the name of the acid is the root of the anion followed by the suffix -ous. Again, there is no prefix. $\ce{HNO_2}$ is nitrous acid because $\ce{NO_2^-}$ is the nitrite ion. Note how the root for a sulfur-containing oxoacid is sulfur- instead of just sulf-. The same is true for a phosphorus-containing oxoacid. The root is phosphor- instead of simply phosph-. Many foods and beverages contain citric acid. Vinegar is a dilute solution of acetic acid. Car batteries contain sulfuric acid that helps in the release of electrons to create electricity. Writing Formulas for Acids Like other compounds that we have studied, acids are electrically neutral. Therefore, the charge of the anion part of the formula must be exactly balanced out by the $\ce{H^+}$ ions. Since $\ce{H^+}$ ions carry a single positive charge, the number of $\ce{H^+}$ ions in the formula is equal to the quantity of negative charge on the anion. Two examples from the table above illustrate this point. The chloride ion carries a $1-$ charge, so only one $\ce{H}$ is needed in the formula of the acid $\left( \ce{HCl} \right)$. The sulfate ion carries a $2-$ charge, so two $\ce{H}$s are needed in the formula of the acid $\left( \ce{H_2SO_4} \right)$. Another way to write the correct formula is to utilize the crisscross method, shown below for sulfuric acid. Summary • Acids are molecular compounds that release hydrogen ions. • A binary acid consists of hydrogen and one other element. • Oxoacids contain hydrogen, oxygen, and one other element. • The name of the acid is based on the anion attached to the hydrogen. • Writing formulas for acids follows the same rules as for binary ionic compounds. Review 1. Define “acid.” 2. What is a binary acid? 3. What is an oxoacid? 4. Name the following acids: 1. H2SO4 2. HCN 3. HCl 4. H3PO4 5. Write formulas for the following acids: 1. hydrobromic acid 2. perchloric acid 3. nitrous acid 7.13: Bases: Naming and Formulas What different things are these workers doing to make soap? Soap making has a long history. Until recently, soap was made using animal fats and lye from wood ashes. The lye served as a base to break down the fats and help form the soap. Needless to say, unless the soap was washed to remove the lye, it was very harsh on the skin. Many families would make their own soap by boiling the lye and fat in a large kettle over an open fire—a long and hot task. Bases A base can be simply defined as an ionic compound that produces hydroxide ions when dissolved in water. One of the most commonly used bases is sodium hydroxide, illustrated below. Names and Formulas of Bases There is no special system for naming bases. Since they all contain the $\ce{OH^-}$ anion, names of bases end in hydroxide. The cation is simply named first. Some examples of names and formulas for bases are shown in the table below. Names and Formulas of Bases Table $1$ Formula Name $\ce{NaOH}$ sodium hydroxide $\ce{Ca(OH)_2}$ calcium hydroxide $\ce{NH_4OH}$ ammonium hydroxide Notice that because bases are ionic compounds, the number of hydroxides in the formula does not affect the name. The compound must be neutral, so the charges of the ions are balanced just as for other ionic compounds. The sodium ion $\left( \ce{Na^+} \right)$ requires one $\ce{OH^-}$ ion to balance the charge, so the formula is $\ce{NaOH}$. Calcium $\left( \ce{Ca^{2+}} \right)$ requires two $\ce{OH^-}$ ions to balance the charge, so the formula is $\ce{Ca(OH)_2}$. Hydroxide ion is a polyatomic ion, and must be in parentheses when there is more than one in a formula. Summary • Bases are ionic compounds that produce hydroxide ions when dissolved in water. • The cation is named first followed by "hydroxide." Review 1. What is a base? 2. What is the charge on the hydroxide anion? 3. Name the following bases: 1. LiOH 2. Mg(OH)2 3. Fe(OH)3 4. Write the formulas for the following bases: 1. nickel (II) hydroxide 2. aluminum hydroxide 3. silver hydroxide	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/07%3A_Chemical_Nomenclature/7.12%3A_Acids_-_Naming_and_Formulas.txt
How do we show electrons in atoms? Diagrams contain a lot of useful information in a compact format. What does the diagram above tell us? The football play diagrammed above describes the lineup of each player on the team and describes how they will move when the ball is snapped. Diagrams of electrons give similar information about where certain electrons are. We can mark these electrons and indicate what happens to them when an element reacts. Electron Dot Diagrams Recall that the valence electrons of an atom are the electrons located in the highest occupied principal energy level. Valence electrons are primarily responsible for the chemical properties of elements. The number of valence electrons can be easily determined from the electron configuration. Several examples from the second period elements are shown in the table below. Table $1$: Element Electron Configuration Number of Valence Electrons lithium $1s^2 \: 2s^1$ 1 beryllium $1s^2 \: 2s^2$ 2 nitrogen $1s^2 \: 2s^2 \: 2p^3$ 5 neon $1s^2 \: 2s^2 \: 2p^6$ 8 In each case, valence electrons are those in the second principal energy level. As one proceeds left to right across a period, the number of valence electrons increases by one. In the $s$ block, Group 1 elements have one valence electron, while Group 2 elements have two valence electrons. In the $p$ block, the number of valence electrons is equal to the group number minus ten. Group 13 has three valence electrons, Group 14 has four, and so on...up through Group 18 with eight. The eight valence electrons, a full outer $s$ and $p$ sublevel, give the noble gases their special stability. When examining chemical bonding, it is necessary to keep track of the valence electrons of each atom. Electron dot diagrams are diagrams in which the valence electrons of an atom are shown as dots distributed around the element's symbol. A beryllium atom, with two valence electrons, has the electron dot diagram below: Since electrons repel each other, the dots for a given atom are distributed evenly around the symbol before they are paired. The table below shows the electron dot diagrams for the entire second period. Table $2$: Electron Dot Diagrams for the Second Period Elements Electron Dot Diagrams for the Second Period Elements Group Number Electron Dot Diagram 1 2 13 14 15 16 17 18 Electron dot diagrams would be the same for each element in the representative element groups. Most transition elements have two valence electrons, though some that have unusual electron configurations have only one. Summary • Electron dot diagrams show the valence electrons for an atom. • The dot diagrams are the same for each element in the representative element groups. Review 1. What are valence electrons primarily responsible for? 2. Calcium would have the same electron dot structure as which element pictured in the table? 3. What is the symbol for an element that would have the same electron dot structure as carbon? 4. Would you expect the group 18 elements to have the same electron dot diagram as neon? 8.02: Octet Rule How are electrons organized in atoms? Graduations are exciting events in one's life. The processional, the ceremonies, even the speeches can be handled because this marks the end of a stage of life and the beginning of another one. The ceremony itself is challenging for those organizing it. There are just enough seats on the stage for the graduates. How do you avoid getting too many people in a row and not enough in the next row? Someone is stationed at the end of the row to count the students as they enter. Only so many are allowed to go into a row, and then you begin to fill the next row. Electrons in atoms behave the same way. There are rules that determine where electrons go in compounds. Octet Rule The noble gases are unreactive because of their electron configurations. The noble gas neon has the electron configuration of $1s^2 \: 2s^2 \: 2p^6$. It has a full outer shell and cannot incorporate any more electrons into the valence shell. The other noble gases have the same outer shell electron configuration, even though they have different numbers of inner-shell electrons. American chemist Gilbert Lewis (1875-1946) used this observation to explain the types of ions and molecules that are formed by other elements. He called his explanation the octet rule. The octet rule states that atoms tend to form compounds in ways that give them eight valence electrons, and thus the electron configuration of a noble gas. An exception to an octet of electrons is in the case of the first noble gas, helium, which only has two valence electrons. This primarily affects the element hydrogen, which forms stable compounds by achieving two valence electrons. Lithium, an alkali metal with three valence electrons, is also an exception to the octet rule. Lithium tends to lose one electron to take on the electron configuration of the nearest noble gas, helium, leaving it with two valence electrons. There are two ways in which atoms can satisfy the octet rule. One way is by sharing their valence electrons with other atoms. The second way is by transferring valence electrons from one atom to another. Atoms of metals tend to lose all of their valence electrons, which leaves them with an octet from the next lowest principal energy level. Atoms of nonmetals tend to gain electrons in order to fill their outermost principal energy level with an octet. Watch the following video to learn how to use the octet rule to predict the charge of an ion. Summary • Atoms form compounds in ways that give them eight valence electrons. • Metals tend to lose electrons to achieve this configuration. • Nonmetals tend to gain electrons to achieve this configuration. Review 1. What is the electron configuration of a noble gas? 2. Why is this configuration important? 3. What are the two ways in which atoms can satisfy the octet rule? 4. How do metals change to obey the octet rule? 5. How do nonmetals change to obey the octet rule? 8.03: Cation Formation How are cations formed? In many parts of the country, the water contains high concentrations of minerals that stain clothes, build up deposits on bathtubs and water heaters, and create problems with soap foaming properly. This problem is caused by "hard water". The water contains excessive amounts of cations such as iron and calcium. These ions create a lot of problems in the water. Ion exchange resins can remove these minerals and clean up the water. Cation Formation Cations are the positive ions formed by the loss of one or more electrons. The most commonly formed cations of the representative elements are those that involve the loss of all of the valence electrons. Consider the alkali metal sodium $\left( \ce{Na} \right)$. It has one valence electron in the third principal energy level. Upon losing that electron, the sodium ion now has an octet of electrons from the second principal energy level. The equation below illustrates this process. $\begin{array}{lcl} \ce{Na} & \rightarrow & \ce{Na^+} + \ce{e^-} \ 1s^2 \: 2s^2 \: 2p^6 \: 3s^1 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ The electron configuration of the sodium ion is now the same as that of the noble gas neon. The term isoelectronic refers to an atom and an ion of a different atom (or two different ions) that have the same electron configuration. The sodium ion is isoelectronic with the neon atom. Consider a similar process with magnesium and with aluminum: $\begin{array}{lcl} \ce{Mg} & \rightarrow & \ce{Mg^{2+}} + 2 \ce{e^-} \ 1s^2 \: 2s^2 \: 2p^6 \: 3s^2 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ $\begin{array}{lcl} \ce{Al} & \rightarrow & \ce{Al^{3+}} + 3 \ce{e^-} \ 1s^2 \: 2s^2 \: 2p^6 \: 3s^2 \: 3p^1 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ In this case, the magnesium atom loses its two valence electrons in order to achieve the same noble gas configuration. The aluminum atom loses its three valence electrons. The $\ce{Mg^{2+}}$ ion, the $\ce{Al^{3+}}$ ion, the $\ce{Na^+}$ ion, and the $ce{Ne}$ atom are all isoelectronic. For representative elements under typical conditions, three electrons are the maximum number that will be lost. We can also show the loss of valence electron(s) with an electron dot diagram: \begin{align} \ce{Na} \cdot &\rightarrow \ce{Na^+} + \ce{e^-} \ \cdot \ce{Mg} \cdot &\rightarrow \ce{Mg^{2+}} + 2 \ce{e^-} \end{align}\nonumber Summary • Cations form when an atom loses one or more electrons. • The resulting cation has the electron configuration of the noble gas atom in the row above it in the periodic table. Review 1. What is a cation? 2. How many valence electrons does the sodium atom have? 3. Which atom is the sodium ion isoelectronic with? 4. How many electrons does magnesium lose to form the magnesium ion?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/08%3A_Ionic_and_Metallic_Bonding/8.01%3A_Electron_Dot_Diagrams.txt
How do you make chlorine safe to eat? How do you transform a deadly gas into something you can sprinkle on your eggs and eat for breakfast? Chlorine in its free form is very dangerous if you breathe the fumes or come in contact with the gas. However, in reaction with sodium, sodium chloride forms as the sodium atom gives up an electron to chlorine, which accepts the electron to form the chloride anion. Anions Anions are the negative ions formed from the gain of one or more electrons. When nonmetal atoms gain electrons, they often do so until their outermost principal energy level achieves an octet. This process is illustrated below for the elements fluorine, oxygen, and nitrogen: $\begin{array}{lcl} \ce{F} + \ce{e^-} & \rightarrow & \ce{F^-} \ 1s^2 \: 2s^2 \: 2p^5 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ $\begin{array}{lcl} \ce{O} + 2 \ce{e^-} & \rightarrow & \ce{O^{2-}} \ 1s^2 \: 2s^2 \: 2p^4 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ $\begin{array}{lcl} \ce{N} + 3 \ce{e^-} & \rightarrow & \ce{N^{3-}} \ 1s^2 \: 2s^2 \: 2p^3 & & 1s^2 \: 2s^2 \: 2p^6 \: \text{(octet)} \end{array}\nonumber$ All of these anions are isoelectronic with each other and with neon. They are also isoelectronic with the three cations from the previous section. Under typical conditions, three electrons are the maximum that will be gained in the formation of anions. Outer electron configurations are constant within a group, so this pattern of ion formation repeats itself for Periods 3, 4, and following (see below). It is important not to misinterpret the concept of being isoelectronic. A sodium ion is very different from a neon atom, because the nuclei of the two contain different numbers of protons. One is an essential ion that is a part of table salt, while the other is an unreactive gas that is a very small part of the atmosphere. Likewise, sodium ions are very different than magnesium ions, fluoride ions, and all the other members of this isoelectronic series $\left( \ce{N^{3-}}, \: \ce{O^{2-}}, \: \ce{F^-}, \: \ce{Ne}, \: \ce{Na^+}, \: \ce{Mg^{2+}}, \: \ce{Al^{3+}} \right)$. Summary • Anions are negative ions formed by accepting electrons. • The outermost principal energy level of an anion is usually an octet. Review 1. What is an anion? 2. Write the electronic configurations for the chlorine atom and the chloride anion. 3. What does isoelectronic mean? 8.05: Transition Metal Ion Formation How do transition metals form ions? The transition metals are an interesting and challenging group of elements. They have perplexing patterns of electron distribution that do not always follow the electron filling rules. Predicting how they will form ions is also not always obvious. Transition Metal Ions Transition metals belong to the $d$ block, meaning that the $d$ sublevel of electrons is in the process of being filled with up to ten electrons. Many transition metals cannot lose enough electrons to attain a noble-gas electron configuration. In addition, the majority of transition metals are capable of adopting ions with different charges. Iron, which forms either the $\ce{Fe^{2+}}$ or $\ce{Fe^{3+}}$ ions, loses electrons as shown below. $\begin{array}{lcl} \ce{Fe} & \rightarrow & \ce{Fe^{2+}} + 2 \ce{e^-} \ \left[ \ce{Ar} \right] \: 3d^6 \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^6 \end{array}\nonumber$ $\begin{array}{lcl} \ce{Fe} & \rightarrow & \ce{Fe^{3+}} + 3 \ce{e^-} \ \left[ \ce{Ar} \right] \: 3d^6 \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^5 \end{array}\nonumber$ According to the Aufbau process, the electrons fill the $4s$ sublevel before beginning to fill the $3d$ sublevel. However, the outermost $s$ electrons are always the first to be removed in the process of forming transition metal cations. Because most transition metals have two valence electrons, the charge of $2+$ is a very common one for their ions. This is the case for iron above. A half-filled $d$ sublevel $\left( d^5 \right)$ is particularly stable, which is the result of an iron atom losing a third electron. Some transition metals that have relatively few $d$ electrons may attain a noble-gas electron configuration. Scandium is an example. $\begin{array}{lcl} \ce{Sc} & \rightarrow & \ce{Sc^{3+}} + 3 \ce{e^-} \ \left[ \ce{Ar} \right] \: 3d^1 \: 4s^2 & & \left[ \ce{Ar} \right] \end{array}\nonumber$ Others may attain configurations with a full $d$ sublevel, such as zinc and copper. $\begin{array}{lcl} \ce{Zn} & \rightarrow & \ce{Zn^{2+}} + 2 \ce{e^-} \ \left[ \ce{Ar} \right] \: 3d^{10} \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^{10} \end{array}\nonumber$ $\begin{array}{lcl} \ce{Cu} & \rightarrow & \ce{Cu^+} + \ce{e^-} \ \left[ \ce{Ar} \right] \: 3d^{10} \: 4s^1 & & \left[ \ce{Ar} \right] \: 3d^{10} \end{array}\nonumber$ The resulting configuration above, with 18 electrons in the outermost principal energy level, is referred to as a pseudo noble-gas electron configuration. It gives particular stability to the $\ce{Zn^{2+}}$ and $\ce{Cu^+}$ ions. Summary • Transition metal ion formation is more complex than simple cation formation. • Transition metal ions often involve rearrangements of both $d$ and $s$ electrons. Review 1. What block do the transition metals fall in? 2. When writing the electron configuration for a transition metal, which sublevel ($s,p,d,f$) is filled first? 3. When forming transition metal ions, which sublevel ($s,p,d,f$) loses electrons first? What is the pseudo noble-gas electron configuration?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/08%3A_Ionic_and_Metallic_Bonding/8.04%3A_Anion_Formation.txt
Does the sea really have salt in it? We can get common table salt from several sources. It can be mined in the solid form in salt mines, or found as a solid in deposits. We can also get salt from the ocean, but it really does not exist as a salt when in solution. The sodium ions and chloride ions are dissolved, but not combined into a structure until all the water is removed. Most of the rocks and minerals that make up the Earth's crust are composed of positive and negative ions held together by ionic bonding. An ionic compound is an electrically neutral compound consisting of positive and negative ions. You are very familiar with some ionic compounds, such as sodium chloride $\left( \ce{NaCl} \right)$. A sodium chloride crystal consists of equal numbers of positive sodium ions $\left( \ce{Na^+} \right)$ and negative chloride ions $\left( \ce{Cl^-} \right)$. Ionic Bonds Oppositely charged particles attract each other. This attractive force is often referred to as an electrostatic force. An ionic bond is the electrostatic force that holds ions together in an ionic compound. The strength of the ionic bond is directly dependent upon the quantity of the charges and inversely dependent on the distance between the charged particles. A cation with a $2+$ charge will make a stronger ionic bond than a cation with a $1+$ charge. A larger ion makes a weaker ionic bond because of the greater distance between its electrons and the nucleus of the oppositely charged ion. Electron Dot Diagrams We will use sodium chloride as an example to demonstrate the nature of the ionic bond and how it forms. As you know, sodium is a metal and loses its one valence electron to become a cation. Chlorine is a nonmetal and gains one electron in becoming an anion. Both achieve a noble-gas electron configuration. However, electrons cannot be simply "lost" to nowhere in particular. A more accurate way to describe what is happening is that a single electron is transferred from the sodium atom to the chlorine atom, as shown below. The ionic bond is the attraction of the $\ce{Na^+}$ ion for the $\ce{Cl^-}$ ion. It is conventional to show the cation without dots around the symbol to emphasize that the original energy level that contained the valence electron is now empty. The anion is now shown with a complete octet of electrons. For a compound such as magnesium chloride, it is not quite as simple. Because magnesium has two valence electrons, it needs to lose both to achieve the noble-gas configuration. Therefore, two chlorine atoms will be needed. The final formula for magnesium chloride is $\ce{MgCl_2}$. Summary • An ionic compound contains positive and negative ions. • An ionic bond is electrostatic in nature. • Electron dot diagrams can be used to illustrate electron movements and ion formation. Review 1. What is an ionic compound? 2. What is an ionic bond? 3. Which cation (Na+ or Ca2+) would form a stronger ionic bond with Cl-? 8.07: Ionic Crystal Structure Why are crystals appealing? Crystals are found everywhere that chemical deposits are located. Ruby crystals are extremely valuable, both because of ruby's beauty and its utility in equipment such as lasers. Some claim that crystals have magical qualities. For others, the "magic" is in the regular structure of the crystal, as the cations and anions line up in a regular order. Ionic Crystal Structure Electron dot diagrams show the nature of the electron transfer that takes place between metal and nonmetal atoms. However, ionic compounds do not exist as discrete molecules, as the dot diagrams may suggest. In order to minimize the potential energy of the system, ionic compounds take on the form of an extended three-dimensional array of alternating cations and anions. This maximizes the attractive forces between the oppositely charged ions. The figure below shows two different ways of representing the ionic crystal lattice. A ball and stick model makes it easier to see how individual ions are oriented with respect to one another. A space filling diagram is a more accurate representation of how the ions pack together in the crystal. Naturally occurring sodium chloride (halite) does not look at first glance like the neat diagrams shown above. It is only when we use modern techniques to analyze the crystal structure at the atomic level that we can see the true regularity of the organized ions. Summary • Ionic compounds take on the form of extended three-dimensional arrays of cations and anions. • The arrangement maximizes the attractive force between oppositely-charged ions. Review 1. Why do ionic compounds form crystal structure instead of discrete molecules? 2. Ions arrange themselves in three-dimensional array to maximize what type of force 3. Which of the following would best describe how ions in an ionic compound arrange themselves? 1. randomly 2. an orderly pattern	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/08%3A_Ionic_and_Metallic_Bonding/8.06%3A_Ionic_Bonding.txt
What makes the colors different? Cobalt salts contain $\ce{Co^{2+}}$ cations. Differences in the color of the salts is due to the species surrounding the cobalt ion. The presence of water molecules in the coordination sphere around the central cobalt ion changes the distances among species and the color of the material. Coordination Number The coordination number is the number of ions that immediately surround an ion of the opposite charge within a crystal lattice. If you examine the figure below, you will see that there are six chloride ions immediately surrounding a single sodium ion. The coordination number of sodium is 6. Likewise, six sodium ions immediately surround each chloride ion, making the coordination number of chloride also equal to 6. Because the formula unit of sodium chloride displays a 1:1 ratio between the ions, the coordination numbers must be the same. The formula unit for cesium chloride is $\ce{CsCl}$, also a 1:1 ratio. However, as shown in the figure below, the coordination numbers are not 6 as in $\ce{NaCl}$. The center ion is the $\ce{Cs^+}$ ion and is surrounded by the eight $\ce{Cl^-}$ ions at the corners of the cube. Each $\ce{Cl^-}$ ion is also surrounded by eight $\ce{Cs^+}$ ions. The coordination numbers in this type of crystal are both 8. $\ce{CsCl}$ and $\ce{NaCl}$ do not adopt identical crystal packing arrangements because the $\ce{Cs^+}$ is considerably larger than the $\ce{Na^+}$ ion. Another type of crystal is illustrated by titanium (IV) oxide, $\ce{TiO_2}$, which is commonly known as rutile. The rutile crystal is shown below. The gray $\ce{Ti^{4+}}$ ions are surrounded by six red $\ce{O^{2-}}$ ions. The $\ce{O^{2-}}$ ions are surrounded by three $\ce{Ti^{4+}}$ ions. The coordination of the titanium (IV) cation is 6, which is twice the coordination number of the oxide anion, which is 3. This fits with the formula unit of $\ce{TiO_2}$, since there are twice as many $\ce{O^{2-}}$ ions as $\ce{Ti^{4+}}$ ions. The crystal structure of all ionic compounds must reflect the formula unit. In a crystal of iron (III) chloride, $\ce{FeCl_3}$, there are three times as many chloride ions as iron (III) ions. Summary • The coordination number of a compound is determined by the type and number of ions or other species surrounding a central ion. • Often the color of a compound is affected by the specific materials coordinated to that central ion. Review 1. What is the coordination number for Na+ in NaCl? 2. What is the coordination number for Cs+? 3. Why are the packing arrangements for Na+ and Cs+ different? 8.09: Physical Properties of Ionic Compounds What produces colored crystals? The figure above shows just a few examples of the color and brilliance of naturally occurring ionic crystals. The regular and orderly arrangement of ions in the crystal lattice is responsible for the various shapes of these crystals, while transition metal ions give rise to the colors. Physical Properties of Ionic Compounds Melting Points Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about $800^\text{o} \text{C}$. Shattering Ionic compounds are generally hard, but brittle. Why? It takes a large amount of mechanical force, such as striking a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens, it brings ions of the same charge next to one another (see below). The repulsive forces between like-charged ions cause the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions. Conductivity Another characteristic property of ionic compounds is their electrical conductivity. The figure below shows three experiments in which two electrodes that are connected to a light bulb are placed in beakers containing three different substances. In the first beaker, distilled water does not conduct a current because water is a molecular compound. In the second beaker, solid sodium chloride also does not conduct a current. Despite being ionic and thus composed of charged particles, the solid crystal lattice does not allow the ions to move between the electrodes. Mobile charged particles are required for the circuit to be complete and the light bulb to light up. In the third beaker, the $\ce{NaCl}$ has been dissolved into the distilled water. Now the crystal lattice has been broken apart and the individual positive and negative ions can move. Cations move to one electrode, while anions move to the other, allowing electricity to flow (see figure below). Melting an ionic compound also frees the ions to conduct a current. Ionic compounds conduct an electric current when melted or dissolved in water. Summary • Ionic compounds have high melting points. • Ionic compounds are hard and brittle. • Solutions of ionic compounds and melted ionic compounds conduct electricity, but solid materials do not. Review 1. Why are ionic compounds brittle? 2. Why are melting points high for ionic compounds? 3. What happens when an electric current is passed through a solution of an ionic compound? Explore More Watch the video below and answer the following questions: 1. Do all ionic compounds form crystals? 2. Will melted ionic compounds conduct electricity? 3. What are the melting and boiling points of KI?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/08%3A_Ionic_and_Metallic_Bonding/8.08%3A_Coordination_Number.txt
Why do metals behave the way they do? The image below is of a copper plate that was made in 1893. The utensil has a great deal of elaborate decoration, and the item is very useful. What would have happened if this plate was made of copper (I) chloride instead? Copper (I) chloride does contain copper, after all. However, the $\ce{CuCl}$ would end up as a powder when a metalworker pounded on it to shape it. Metals behave in unique ways. The bonding that occurs in a metal is responsible for its distinctive properties: luster, malleability, ductility, and excellent conductivity. The Metallic Bond Pure metals are crystalline solids, but unlike ionic compounds, every point in the crystal lattice is occupied by an identical atom. The electrons in the outer energy levels of a metal are mobile and capable of drifting from one metal atom to another. This means that the metal is more properly viewed as an array of positive ions surrounded by a sea of mobile valence electrons. Electrons which are capable of moving freely throughout the empty orbitals of the metallic crystal are called delocalized electrons (see below). A metallic bond is the attraction of the stationary metal cations to the surrounding mobile electrons. Properties of Metals The metallic bonding model explains the physical properties of metals. Metals conduct electricity and heat very well because of their free-flowing electrons. As electrons enter one end of a piece of metal, an equal number of electrons flow outward from the other end. When light is shone onto the surface of a metal, its electrons absorb small amounts of energy and become excited into one of its many empty orbitals. The electrons immediately fall back down to lower energy levels and emit light. This process is responsible for the high luster of metals. Recall that ionic compounds are very brittle. Application of a force results in like-charged ions in the crystal coming too close to one another, causing the crystal to shatter. When a force is applied to a metal, the free-flowing electrons can slip in between the stationary cations and prevent them from coming in contact. Imagine ball bearings that have been coated with oil sliding past one another. As a result, metals are very malleable and ductile. They can be hammered into shapes, rolled into thin sheets, or pulled into thin wires. Summary • The metallic bond is responsible for the properties of metals. • Metals conduct electricity and heat well. • Metals are ductile and malleable. • Metals have luster. Review 1. What is a delocalized electron? 2. Why do metals conduct electricity and heat well? 3. Why do metals have luster? 8.11: Crystal Structure of Metals How would you stack cannon balls? Before modern artillery with explosive shells, cannons were used to fire cannon balls at the enemy. The soldiers operating the cannon needed to be able to get to the cannon balls quickly and efficiently. A pyramidal arrangement worked well for this purpose. Crystal Structures of Metals When identical spheres are stacked, each successive layer fits into the small spaces where different spheres come together. This orderly and regular arrangement of the metal balls minimizes the empty space between them. Closest packing is the most efficient arrangement of spheres. Atoms of a metal crystal are arranged in similar patterns, called close-packed structures. Pure metals adopt one of several related close-packed structures, as shown below. On the far left is the body-centered cubic (bcc) structure. In that crystal, metal atoms occupy the eight corners of a cube along with one atom in the very center. The coordination number of each atom in the body-centered cubic structure is 8. In the face-centered cubic (fcc) structure, there are eight atoms at each corner of the cube and six atoms in the center of each face. The coordination number of each atom in the face-centered cubic structure is 12. The hexagonal close-packed (hcp) structure also has a coordination number of 12, but crystals of this type are hexagonally shaped rather than cubic. Summary • Atoms of a metal crystal are arranged in close-packed structures. • This type of structure minimizes the empty space between the atoms. Review 1. What is the most efficient arrangement of spheres? 2. What is the coordination number of a face-centered cubic structure? 3. What other structure has a coordination number of 12? 8.12: Alloys What are the best guitar strings to use? Many guitar players are very meticulous when it comes to their guitar strings. There is a variety to select from, dependent on the type of guitar and the style of music. Electric guitars need steel strings so that the magnetic pick-up will detect the string vibrations. Acoustic guitar players have several choices; bronze strings (mixed with different amounts of copper and zinc) have perhaps the brightest tone. There are several combinations of bronze alloys to choose from. For those with a high budget, titanium strings are available. Gold coating also helps string life and makes its unique contribution to tone. Alloy chemistry has contributed greatly to the strength, durability, and tonal quality of guitar strings. Alloys An alloy is a mixture composed of two or more elements, at least one of which is a metal. You are probably familiar with some alloys such as brass and bronze. Brass is an alloy of copper and zinc. Bronze is an alloy of copper and tin. Alloys are commonly used in manufactured items because the properties of these metal mixtures are often superior to a pure metal. Bronze is harder than copper and more easily cast. Brass is very malleable and its acoustic properties make it useful for musical instruments. Steels are a very important class of alloys. The many types of steels are primarily composed of iron, with various amounts of the elements carbon, chromium, manganese, nickel, molybdenum, and boron. Steels are widely used in building construction because of their strength, hardness, and resistance to corrosion. Most large modern structures like skyscrapers and stadiums are supported by a steel skeleton (see figure below). Alloys can be one of two general types. In one type, called a substitutional alloy, the various atoms simply replace each other in the crystal structure. In another type, called an interstitial alloy, the smaller atoms such as carbon fit in between the larger atoms in the crystal packing arrangement. Summary • Alloys are mixtures of materials, at least one of which is a metal. • Bronze alloys were widely used in weapons. • Brass alloys have long been employed in musical instruments. • Steel alloys are strong and durable. Review 1. What is brass made of? 2. What is bronze made of? 3. Why is steel widely used in construction? 4. What is a substitutional alloy?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/08%3A_Ionic_and_Metallic_Bonding/8.10%3A_Metallic_Bonding.txt
Molecules are groups of atoms that behave as a single unit. Some elements exist as molecules: hydrogen, oxygen, sulfur, and so forth. There are rules that can express a unique name for any given molecule, and a unique formula for any given name. 09: Covalent Bonding Have you ever made cupcakes from scratch? You mix together flour, sugar, eggs, and other ingredients to make the batter, put the batter into cupcake papers, and then put them into the oven to bake. The cupcakes that come out of the oven after baking are different from any of the individual ingredients that went into the batter. Like the ingredients that join together to make cupcakes, atoms of different elements can join together to form entirely different substances called compounds. In cupcakes, the eggs and other wet ingredients cause the dry ingredients to stick together. What causes elements to stick together in compounds? The answer is chemical bonds. What Is a Chemical Bond? A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds. Q: Why do you think that chemical bonds form? A: Chemical bonds form because they give atoms a more stable arrangement of electrons. Why Bonds Form To understand why chemical bonds form, consider the common compound(opens in new window) known as water, or H2O. It consists of two hydrogen (H) atoms and one oxygen (O) atom. As you can see in the on the left side of the figure below, each hydrogen atom has just one electron, which is also its sole valence electron. The oxygen atom has six valence electrons. These are the electrons in the outer energy level of the oxygen atom. In the water molecule on the right in the figure above, each hydrogen atom shares a pair of electrons with the oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. The hydrogen atoms each have a pair of shared electrons, so their first and only energy level is full. The oxygen atom has a total of eight valence electrons, so its outer energy level is full. A full outer energy level is the most stable possible arrangement of electrons. It explains why elements form chemical bonds with each other. Types of Chemical Bonds Not all chemical bonds form in the same way as the bonds in water. There are actually three different types of chemical bonds, called covalent, ionic, and metallic bonds. Each type of bond is described below. • A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented in Figure $2$ contains covalent bonds. • An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals instead of molecules. Table salt contains ionic bonds. • A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround it—both its own valence electrons and those of other ions of the same metal. The ions and electrons form a lattice-like structure. Only metals, such as the copper pictured in Figure $3$, form metallic bonds. Summary • A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. • Atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons. • There are three different types of chemical bonds: covalent, ionic, and metallic bonds. Review 1. What is a chemical bond? 2. Explain why hydrogen and oxygen atoms are more stable when they form bonds in a water molecule. 3. How do ionic bonds and covalent bonds differ? Explore More Watch this video about covalent bonds, and then answer the questions below. 1. Which types of elements can form covalent bonds? 2. How can you tell the number of covalent bonds an element can form? 3. Why does one atom of nitrogen form bonds with three atoms of hydrogen?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.01%3A_Chemical_Bond.txt
In a tennis match, two players keep hitting the ball back and forth. The ball bounces from one player to the other, over and over again. The ball keeps the players moving together on the court. What if the two players represented the nuclei of two atoms and the ball represented valence electrons? What would the back and forth movement of the ball represent? The answer is a covalent bond. Sharing Electrons A covalent bond is the force of attraction that holds together two atoms that share a pair of valence electrons. The shared electrons are attracted to the nuclei of both atoms. This forms a molecule consisting of two or more atoms. Covalent bonds form only between atoms of nonmetals. Covalent Compounds and Diatomic Elements The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the figure below. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the figure below shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so it’s not a new compound. Oxygen normally occurs in diatomic (“two-atom”) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. Why Covalent Bonds Form Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the oxygen atoms in the figure above. Alone, each oxygen atom has six valence electrons. By sharing two pairs of valence electrons, each oxygen atom has a total of eight valence electrons. This fills its outer energy level, giving it the most stable arrangement of electrons. The shared electrons are attracted to both oxygen nuclei, and this force of attraction holds the two atoms together in the oxygen molecule. Summary • A covalent bond is the force of attraction that holds together two atoms that share a pair of valence electrons. Covalent bonds form only between atoms of nonmetals. • The two atoms that are held together in a covalent bond may be atoms of the same element or different elements. When atoms of different elements bond together, it forms a covalent compound. • Covalent bonds form because the shared electrons fill each atom’s outer energy level and this is the most stable arrangement of electrons. Review 1. What is a covalent bond? 2. Nitrogen is a diatomic element with five valence electrons. Create a model of a molecule of nitrogen. 3. Which of the following represents a covalent compound? 1. O2 2. CO2 3. Cl2 4. NaCl Explore More Watch the video about covalent bonding and then answer the questions below. 1. How can you tell the number covalent bonds the atoms of an element can form? 2. How many covalent bonds can nitrogen form? How many covalent bonds can chlorine form? 3. Why are covalent bonds stronger than ionic bonds? 4. Why can’t molecules of a covalent compound conduct electricity?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.02%3A_Covalent_Bond.txt
The burner on a gas stove burns with a pretty blue flame like the one pictured in the opening image. The fuel burned by most gas stoves is natural gas, which consists mainly of methane. Methane is a compound that contains only carbon and hydrogen. Like many other compounds that consist of just these two elements, methane is used for fuel because it burns very easily. Methane is an example of a covalent compound. What Are Covalent Compounds? Compounds that form from two or more nonmetallic elements, such as carbon and hydrogen, are called covalent compounds. In a covalent compound, atoms of the different elements are held together in molecules by covalent bonds. These are chemical bonds in which atoms share valence electrons. The force of attraction between the shared electrons and the positive nuclei of both atoms holds the atoms together in the molecule. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates, which are compounds in living things. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom, as you can see in the figure below. Naming and Writing Formulas for Covalent Compounds To name simple covalent compounds, follow these rules: • Start with the name of the element closer to the left side of the periodic table. • Follow this with the name of element closer to the right of the periodic table. Give this second name the suffix –ide. • Use prefixes to represent the numbers of the different atoms in each molecule of the compound. The most commonly used prefixes are shown in the Table below. Table $1$ Naming and Writing Formulas for Covalent Compounds Number Prefix 1 mono- (or none) 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- Q: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? A: The compound is named dinitrogen trioxide. Nitrogen is named first because it is farther to the left in the periodic table than oxygen. Oxygen is given the -ide suffix because it is the second element named in the compound. The prefix di- is added to nitrogen to show that there are two atoms of nitrogen in each molecule of the compound. The prefix tri- is added to oxygen to show that there are three atoms of oxygen in each molecule. In the chemical formula for a covalent compound, the numbers of the different atoms in a molecule are represented by subscripts. For example, the formula for the compound named carbon dioxide is CO2. Q: What is the chemical formula for dinitrogen trioxide? A: The chemical formula is N2O3. Properties of Covalent Compounds The covalent bonds of covalent compounds are responsible for many of the properties of the compounds. Because valence electrons are shared in covalent compounds, rather than transferred between atoms as they are in ionic compounds, covalent compounds have very different properties than ionic compounds. • Many covalent compounds, especially those containing carbon and hydrogen, burn easily. In contrast, many ionic compounds do not burn. • Many covalent compounds do not dissolve in water, whereas most ionic compounds dissolve well in water. • Unlike ionic compounds, covalent compounds do not have freely moving electrons, so they cannot conduct electricity. • The individual molecules of covalent compounds are more easily separated than the ions in a crystal, so most covalent compounds have relatively low boiling points. This explains why many of them are liquids or gases at room temperature. You can compare the boiling points of some covalent and ionic compounds in the Table below. Table $2$ Properties of Covalent Compounds Name of Compound(Chemical Formula) Type of Compound Boiling Point (°C) Methane (CH4) covalent -164 Nitrogen oxide (NO) covalent -152 Sodium chloride (NaCl) ionic 1413 Lithium fluoride (LiF) ionic 1676 Q: The two covalent compounds in the table are gases at room temperature, which is 20°C. For a compound to be a liquid at room temperature, what does its boiling point have to be? A: To be a liquid at room temperature, a covalent compound has to have a boiling point higher than 20°C. Water is an example of a covalent compound that is a liquid at room temperature. The boiling point of water is 100°C. Summary • Covalent compounds contain two or more nonmetallic elements held together by covalent bonds, in which atoms share pairs of valence electrons. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. • A few rules can be applied to name simple covalent compounds: name the left-most element in the periodic table first, add -ide to the second named element, and use prefixes for the numbers of atoms. • Covalent bonds are responsible for many of the properties of covalent compounds. Covalent compounds have relatively low boiling points, cannot conduct electricity, and may not dissolve in water. Review 1. What are covalent compounds? Give two examples. 2. What is the name of the covalent compound that consists of two atoms of silicon and six atoms of bromine? What is its chemical formula? 3. Compare and contrast the properties of ionic and covalent compounds.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.03%3A_Molecular_Compounds.txt
Why do BeCl2 and LiCl bond differently? We have learned that halide salts of elements in group 1 are typically ionic compounds. We would expect $\ce{LiCl}$ to exist as $\ce{Li^+}$ cations and $\ce{Cl^-}$ anions (and it does). However, if we move one column to the right, lithium's neighbor beryllium forms a different type of bond altogether. This bond consists of shared electrons between the $\ce{Be}$ and $\ce{Cl}$ atoms, not electrostatic attraction among ions. Energy and Covalent Bond Formation Molecular compounds are those that take the form of an individual molecule. Molecular compounds are generally comprised of two or more nonmetal atoms. Familiar examples include water $\left( \ce{H_2O} \right)$, carbon dioxide $\left( \ce{CO_2} \right)$, and ammonia $\left( \ce{NH_3} \right)$. Recall that the molecular formula shows the number of each atom that occurs in a molecule of that compound. One molecule of water contains two hydrogen atoms and one oxygen atom. Hydrogen $\left( \ce{H_2} \right)$ is an example of an element that exists naturally as a diatomic molecule. A diatomic molecule is a molecule containing two atoms. Most atoms attain a lower potential energy when they are bonded to other atoms than when they are separated. Consider two isolated hydrogen atoms that are separated by a distance large enough to prevent any interaction between them. At this distance, the potential energy of the system is said to be equal to zero (see figure below). As the atoms approach one another, their electron clouds gradually begin to overlap. Now there are several interactions which begin to occur. One is that the single electrons that each hydrogen atom possesses begin to repel each other. This repulsive force would tend to make the potential energy of the system increase. However, the electron of each atom begins to be attracted to the nucleus of the other atom. This attractive force tends to make the potential energy of the system decrease. As the atoms first begin to interact, the attractive force is stronger than the repulsive force and so the potential energy of the system decreases, as seen in the diagram. Remember that the lower potential energy increases the stability of the system. As the two hydrogen atoms move closer and closer together, the potential energy is at its lowest possible point. If the hydrogen atoms move any closer together, a third interaction begins to dominate, and that is the repulsive force between the two positively-charged nuclei. This repulsive force is very strong, as can be seen by the sharp rise in energy at the far left of the diagram. The point at which the potential energy reached its minimum represents the ideal distance between hydrogen atoms for a stable chemical bond to occur. This type of chemical bond is called a covalent bond. A covalent bond is a bond in which two atoms share one or more pairs of electrons. The single electrons from each of the two hydrogen atoms are shared when the atoms come together to form a hydrogen molecule $\left( \ce{H_2} \right)$. Summary • Covalent bonds are formed when atoms share electrons between them. Review 1. What is a diatomic molecule? 2. What does lower potential energy do to a system? 3. What is the ideal distance for two atoms?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.04%3A_Energy_and_Covalent_Bond_Formation.txt
What does cholesterol really look like? We can write the structure of the cholesterol molecule a couple of different ways. The simplest approach is to simply write $\ce{C_{27}H_{46}O}$. This "structure" is not very useful because it does not tell us how the carbons, hydrogens, and oxygen are connected to one another. The structure in the figure above is much more helpful—we see how the different atoms are connected together to form the molecule. Lewis Electron-Dot Structures In a previous chapter, you learned that the valence electrons of an atom can be shown in a simple way with an electron dot diagram. A hydrogen atom is shown as $\ce{H} \cdot$ because of its one valence electron. The structures of molecules that are held together by covalent bonds can be diagrammed by Lewis electron-dot structures. The hydrogen molecule is shown in the figure below. The shared pair of electrons is shown as two dots in between the two $\ce{H}$ symbols $\left( \ce{H:H} \right)$. This is called a single covalent bond, when two atoms are joined by the sharing of one pair of electrons. The single covalent bond can also be shown by a dash in between the two symbols $\left( \ce{H-H} \right)$. A structural formula is a formula that shows the arrangement of atoms in a molecule and represents covalent bonds between atoms by dashes. The Octet Rule and Covalent Bonds When ions form, they conform to the octet rule by either losing or gaining electrons in order to achieve the electron configuration of the nearest noble gas. In a similar way, nonmetal atoms share electrons in the formation of a covalent bond in such a way that each of the atoms involved in the bond can attain a noble-gas electron configuration. The shared electrons are "counted" for each of the atoms involved in the sharing. For hydrogen $\left( \ce{H_2} \right)$, the shared pair of electrons means that each of the atoms is able to attain the electron configuration of helium, the noble gas with two electrons. For atoms other than hydrogen, the sharing of electrons will usually provide each of the atoms with eight valence electrons. Summary • Lewis electron-dot structures show the bonding in covalent molecules. • Covalent bonds between atoms can be indicated either with dots $\left( \ce{:} \right)$ or a dash $\left( \ce{-} \right)$. Review 1. What is a single covalent bond? 2. How can covalently-bound atoms obey the octet rule? 3. Does the hydrogen molecule obey the octet rule? 9.06: Single Covalent Bonds What holds molecules together? In one form or another, the idea of atoms connecting to form larger substances has been with us for a long time. The Greek philosopher Democritus (460-370 BCE) believed that atoms had hooks that allowed them to connect with one another. Today, we believe that atoms are held together by bonds formed when two atoms share a set of electrons—a much more complicated picture than the simple hooks that Democritus believed in. Single Covalent Bonds A covalent bond forms when two orbitals with one electron each overlap one another. For the hydrogen molecule, this can be shown as: Upon formation of the $\ce{H_2}$ molecule, the shared electrons must have opposite spin, so they are shown with opposite spin in the atomic $1s$ orbital. The halogens also form single covalent bonds in their diatomic molecules. An atom of any halogen, such as fluorine, has seven valence electrons. Its unpaired electron is located in the $2p$ orbital. The single electrons in the third $2p$ orbital combine to form the covalent bond: The diatomic fluorine molecule $\left( \ce{F_2} \right)$ contains a single shared pair of electrons. Each $\ce{F}$ atom also has three pair of electrons that are not shared with the other atom. A lone pair is a pair of electrons in a Lewis electron-dot structure that is not shared between atoms. The oxygen atom in the water molecule shown below has two lone pair sets of electrons. Each $\ce{F}$ atom has three lone pairs. Combined with the two electrons in the covalent bond, each $\ce{F}$ atom follows the octet rule. Example $1$ Draw the Lewis electron dot structure for water. Known • Molecular formula of water $= \ce{H_2O}$ • $1 \: \ce{O}$ atom $= 6$ valence electrons • $2 \: \ce{H}$ atoms $=2 \times 1 = 2$ valence electrons • Total number of valence electrons $=8$ Use the periodic table to determine the number of valence electrons for each atom and the total number of valence electrons. Arrange the atoms and distribute the electrons so that each atom follows the octet rule. The oxygen atom will have 8 electrons, while the hydrogen atoms will each have 2. Step 2: Solve. Electron dot diagrams for each atom are: Each hydrogen atom with its single electron will form a covalent bond with the oxygen atom where it has a single electron. The resulting Lewis electron dot structure is: Step 3: Think about your result. The oxygen atom follows the octet rule with two pairs of bonding electrons and two lone pairs. Each hydrogen atom follows the octet rule with one bonding pair of electrons. Summary • Covalent bonds form when electrons in two atoms form overlapping orbitals. • Lone pair electrons in an atom are not shared with any other atom. Review 1. How does a covalent bond form? 2. What relationship do the spins of shared electrons have with each other? 3. Do lone pair electrons form covalent bonds?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.05%3A_Lewis_Electron-Dot_Structures.txt
What do you do with your leftovers? When working with covalent structures, it sometimes looks like there are leftover electrons. Maybe you have applied the rules you learned so far, but there are still some electrons hanging out there, unattached. You can't just leave them there...So, where do you put them? Multiple Covalent Bonds Some molecules are not able to satisfy the octet rule by making only single covalent bonds between the atoms. Consider the compound ethene, which has a molecular formula of $\ce{C_2H_4}$. The carbon atoms are bonded together, with each carbon also bonded to two hydrogen atoms. two $\ce{C}$ atoms $= 2 \times 4 = 8$ valence electrons four $\ce{H}$ atoms $= 4 \times 1 = 4$ valence electrons = total of 12 valence electrons in the molecule If the Lewis electron dot structure was drawn with a single bond between the carbon atoms and with the octet rule followed, it would look like this: This Lewis structure is incorrect because it contains a total of 14 electrons. However, the Lewis structure can be changed by eliminating the lone pairs on the carbon atoms and having the carbon atoms share two pairs, instead of only one pair, of electrons. A double covalent bond is a covalent bond formed by atoms that share two pairs of electrons. The double covalent bond that occurs between the two carbon atoms in ethane can also be represented by a structural formula and with a molecular model, as shown in the figure below. A triple covalent bond is a covalent bond formed by atoms that share three pairs of electrons. The element nitrogen is a gas that composes the majority of Earth's atmosphere. A nitrogen atom has five valence electrons, which can be shown as one pair and three single electrons. When combining with another nitrogen atom to form a diatomic molecule, the three single electrons on each atom combine to form three shared pairs of electrons. Each nitrogen atom follows the octet rule with one lone pair of electrons, and six electrons that are shared between the atoms. Summary • Lewis structures can be drawn for molecules that share multiple pairs of electrons. • A double covalent bond is a covalent bond formed by atoms that share two pairs of electrons. • A triple covalent bond is a covalent bond formed by atoms that share three pairs of electrons. Review 1. Why is the first ethene Lewis structure incorrect? 2. What do the single electrons in nitrogen do to form a triple bond? 3. Draw the Lewis structure for ethyne C2H2. 9.08: Coordinate Covalent Bond Is sharing a good thing? Remember when you were younger, and were told to share your favorite toy with your brother, sister, or friend? You probably didn't want to share, but did anyway. It likely turned out that you had more fun playing with the toy together than if you had kept it to yourself. Atoms often share electrons with other atoms that have nothing to contribute to the situation; the end result is a new structure. Coordinate Covalent Bonds Each of the covalent bonds that we have looked at so far has involved each of the atoms that are bonding contributing one of the electrons to the shared pair. There is an alternate type of covalent bond in which one of the atoms provides both of the electrons in a shared pair. Carbon monoxide, $\ce{CO}$, is a toxic gas that is released as a byproduct during the burning of fossil fuels. The bonding between the $\ce{C}$ atom and the $\ce{O}$ atom can be thought of in the following procession: At this point, a double bond has formed between the two atoms, with each atom providing one of the electrons to each bond. The oxygen atom now has a stable octet of electrons, but the carbon atom only has six electrons and is unstable. This situation is resolved if the oxygen atom contributes one of its lone pairs in order to make a third bond with the carbon atom. The carbon monoxide molecule is correctly represented by a triple covalent bond between the carbon and oxygen atoms. One of the bonds is a coordinate covalent bond, a covalent bond in which one of the atoms contributes both of the electrons in the shared pair. Once formed, a coordinate covalent bond is the same as any other covalent bond. It is not as if the two conventional bonds in the $\ce{CO}$ molecule are stronger or different in any other way than the coordinate covalent bond. Summary • Coordinate covalent bonds can form when one atom provides a lone pair of electrons to the bond. • Coordinate covalent bonds are as strong as other covalent bonds. Review 1. Where does the third covalent bond in the CO molecule come from? 2. Why is the incorrect structure for CO above wrong? 3. Are coordinate covalent bonds stronger or weaker than regular covalent bonds?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.07%3A_Multiple_Covalent_Bonds.txt
How do we extend basic principles? The United States Supreme Court has the unenviable task of determining what the law dictates. This responsibility can be a major challenge when there is no clear principle involved, or when they face a situation not previously encountered. Chemistry faces the same challenge in extending basic concepts to fit a new situation. Drawing of Lewis structures for polyatomic ions uses the same approach, but tweaks the process a little to fit a somewhat different set of circumstances. Polyatomic Ions Recall that a polyatomic ion is a group of atoms that are covalently bonded together, and which carry an overall electrical charge. The ammonium ion, $\ce{NH_4^+}$, is formed when a hydrogen ion $\left( \ce{H^+} \right)$ attaches to the lone pair of an ammonia $\left( \ce{NH_3} \right)$ molecule in a coordinate covalent bond. When drawing the Lewis structure of a polyatomic ion, the charge of the ion is reflected in the number of total valence electrons in the structure. In the case of the ammonium ion: $1 \: \ce{N}$ atom $= 5$ valence electrons $4 \: \ce{H}$ atoms $= 4 \times 1 = 4$ valence electrons subtract 1 electron for the $1+$charge of the ion total of 8 valence electrons in the ion It is customary to put the Lewis structure of a polyatomic ion into a large set of brackets, with the charge of the ion as a superscript outside of the brackets. Example $1$ Draw the Lewis electron dot structure for the sulfate ion. Known Molecular formula of sulfate ion: $\ce{SO_4^{2-}}$ $1 \: \ce{S}$ atom $= 6$ valence electrons $4 \: \ce{O}$ atoms $= 4 \times 6 = 24$ valence electrons add 2 electrons for the $2-$ charge of the ion = total of 32 valence electrons The less electronegative sulfur atom is the central atom in the structure. Place the oxygen atoms around the sulfur atom, each with a single covalent bond. Distribute lone pairs to each oxygen atom in order to satisfy the octet rule. Count the total number of atoms. If there are too many electrons in the structure, make multiple bonds between the $\ce{S}$ and $\ce{O}$. Step 3: Think about your result. The Lewis structure for the sulfate ion consists of a central sulfur atom with four single bonds to oxygen atoms. This yields the expected total of 32 electrons. Since the sulfur atom started with six valence electrons, two of the $\ce{S-O}$ bonds are coordinate covalent. Summary • Lewis structures for polyatomic ions follow the same rules as those for other covalent compounds. Review 1. What are two characteristics of polyatomic ions? 2. Which atom becomes the central atom in the structure? 3. Where is the charge on an ion placed in a lewis dot diagram? 9.10: Resonance Is this a reflection or a doorway? You may look at the picture below and think you are looking at the image of a room as reflected in a mirror (and you probably are). But the picture can be cropped in such a way as to give the impression of looking at the real room through a door. You would see the same thing and receive the same information, but it would be from a different perspective. There are molecules that can be represented in different ways, such that reality becomes a matter of interpretation. Resonance There are some cases in which more than one viable Lewis structure can be drawn for a molecule. An example is the ozone $\left( \ce{O_3} \right)$ molecule in the figure below. There are a total of 18 electrons in the structure, and so the following two structures are possible: The structure on the left (see figure above) can be converted to the structure on the right by a shifting of electrons without altering the positions of the atoms. It was once thought that the structure of a molecule such as $\ce{O_3}$ consisted of one single bond and one double bond which then shifted back and forth as shown above. However, further studies showed that the two bonds are identical. Any double covalent bond between two given atoms is typically shorter than a single covalent bond. Studies of the $\ce{O_3}$ and other similar molecules showed that the bonds were identical in length. Interestingly, the length of the bond is in between the lengths expected for an $\ce{O-O}$ single bond and a double bond. Resonance is the use of two or more Lewis structures to represent the covalent bonding in a molecule. One of the valid structures is referred to as a resonance structure. It is now understood that the true structure of a molecule which displays resonance is that of an average or a hybrid of all the resonance structures. In the case of the $\ce{O_3}$ molecule, each of the covalent bonds between $\ce{O}$ atoms are best thought of as being "one and a half" bonds, as opposed to either a pure single bond or a pure double bond. This "half-bond" can be shown as a dotted line in both the Lewis structure and the molecular model (see figure below). Many polyatomic ions also display resonance. In some cases, the true structure may be an average of three valid resonance structures, as in the case of the nitrate ion, $\ce{NO_3^-}$ (see figure below). The bond lengths between the central $\ce{N}$ atom and each $\ce{O}$ atom are identical and the bonds can be approximated as being equal to one and one-third bonds. Summary • Resonance structures are averages of different Lewis structure possibilities. • Bond lengths are intermediate between covalent bonds and covalent double bonds. Review 1. How many electrons total are in the ozone structure? 2. What is changed in the two resonance structures of ozone? 3. How can we think of the covalent bonds in ozone?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.09%3A_Covalent_Bonding_in_Polyatomic_Ions.txt
Are rules always followed? Every spring, millions of Americans file their income tax forms. The different rules determine how much tax a person pays. There are also exceptions to the rules. You pay less tax if you are married and/or have children. There are certain limits on how much money you can make before paying taxes. The rule is that you pay taxes, but there are also exceptions based on your personal situation. The bonding rules for molecules are generally applicable, but there are some exceptions allowed. Exceptions to the Octet Rule As the saying goes, all rules are made to be broken. When it comes to the octet rule, this is true. Exceptions to the octet rule fall into one of three categories: (1) an incomplete octet, (2) odd-electron molecules, and (3) an expanded octet. Incomplete Octet In some compounds, the number of electrons surrounding the central atom in a stable molecule is fewer than eight. Beryllium is an alkaline earth metal and so may be expected to form ionic bonds. However, its very small size and somewhat higher ionization energy compared to other metals actually leads beryllium to form primarily molecular compounds. Since beryllium only has two valence electrons, it does not typically attain an octet through sharing of electrons. The Lewis structure of gaseous beryllium hydride $\left( \ce{BeH_2} \right)$ consists of two single covalent bonds between $\ce{Be}$ and $\ce{H}$ (see figure below). Boron and aluminum, with three valence electrons, also tend to form covalent compounds with an incomplete octet. The central boron atom in boron trichloride $\left( \ce{BCl_3} \right)$ has six valence electrons, as shown in the figure below. Odd-Electron Molecules There are a number of molecules whose total number of valence electrons is an odd number. It is not possible for all of the atoms in such a molecule to satisfy the octet rule. An example is nitrogen dioxide $\left( \ce{NO_2} \right)$. Each oxygen atom contributes six valence electrons and the nitrogen atom contributes five for a total of seventeen. The Lewis structure for $\ce{NO_2}$ appears in the figure below. Expanded Octets Atoms of the second period cannot have more than eight valence electrons around the central atom. However, atoms of the third period and beyond are capable of exceeding the octet rule by having more than eight electrons around the central atom. Starting with the third period, the $d$ sublevel becomes available, so it is possible to use these orbitals in bonding, resulting in an expanded octet. Phosphorus and sulfur are two elements that react with halogen elements and make stable compounds with expanded octets. In phosphorus pentachloride, the central phosphorus atom makes five single bonds to chlorine atoms and, as a result, has ten electrons surrounding it (see figure below). In sulfur hexafluoride, the central sulfur atom has twelve electrons from its six bonds to fluorine atoms (see figure below). Summary • Exceptions exist to the rules for covalent bonding • These exceptions apply to atoms whose electrons will not accommodate the normal octet rule. Review 1. What is an incomplete octet? 2. What is an odd-electron molecule? 3. Why are there extra electrons in the expanded octet?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.11%3A_Exceptions_to_the_Octet_Rule.txt
How does smog form? We know that nitrogen makes up about $79\%$ of the gases in the air, and that this gas is chemically very inert. However, we also know that a major contributor to the production of smog is a mixture of nitrogen compounds referred to as $\ce{NO}_x$. Due to the high combustion temperatures in modern car engines (brought about by the need for better fuel efficiency), we make more $\ce{NO}_x$ that will react with other materials to create smog. So, our inert nitrogen gas can be converted to other compounds if enough energy is available to break the triple bonds in the $\ce{N_2}$ molecule. Bond Energy The formation of a chemical bond results in a decrease in potential energy. Consequently, breaking a chemical bond requires an input of energy. Bond energy is the energy required to break a covalent bond between two atoms. A high bond energy means that a bond is strong and the molecule that contains that bond is likely to be stable and less reactive. More reactive compounds will contain bonds that have generally lower bond energies. Some bond energies are listed in the table below. Bond Bond Energy $\left( \text{kJ/mol} \right)$ Table $1$: Bond Energies $\ce{H-H}$ 436 $\ce{C-H}$ 414 $\ce{C-C}$ 347 $\ce{C=C}$ 620 $\ce{C \equiv C}$ 812 $\ce{F-F}$ 157 $\ce{Cl-Cl}$ 243 $\ce{Br-Br}$ 193 $\ce{I-I}$ 151 $\ce{N \equiv N}$ 941 The halogen elements all exist naturally as diatomic molecules ($\ce{F_2}$, $\ce{Cl_2}$, $\ce{Br_2}$, and $\ce{I_2}$). They are generally very reactive and thus have relatively low bond energies. As can be seen by a comparison of the bond energies for the various carbon-carbon bonds, double bonds are substantially stronger than single bonds. Likewise, triple bonds are even stronger. The triple bond that exists between the nitrogen atoms in nitrogen gas $\left( \ce{N_2} \right)$ makes it very unreactive. All plants and animals require the element nitrogen, but it cannot be obtained from the direct absorption of nitrogen gas from the atmosphere because of its strong, unreactive triple bond. Instead, bacteria convert the nitrogen to a more usable form such as ammonium and nitrate ions, which are then absorbed by plants from the soil. Animals only obtain nitrogen by eating those plants. Summary • Bond energy is an indication of the amount of energy needed for a chemical reaction. • The higher the bond energy, the less reactive the bond is. Review 1. What does a high bond energy mean? 2. What kind of bonds do more reactive compounds have? 3. Which will react more readily: a C-H bond or a Cl-Cl bond? 9.13: VSEPR Theory Why is the water molecule bent like that? The characteristic bent shape of the water molecule shown above was a puzzling discovery for scientists at first. The shape allows the molecule to be polar, increasing its boiling point and making it possible for life on earth to exist as we know it. But what makes it bend? The structure is almost the same as carbon dioxide which is known to be a gas at room temperature, why not water too? Putting atoms together to form compounds can be done on paper or in the lab. However, when the shape of the molecule made in the lab is different from the shape of the molecule drawn on paper, then we need to rethink our ideas and find better explanations. VSEPR Theory In 1956, British scientists R.J. Gillespie and R.S. Nyholm recognized that the current model for explaining bond angles did not work well. The theory at that time relied on hybrid orbitals to explain all aspects of bonding. The problem was that this theory gave an incorrect prediction of bond angles for many compounds. They developed a new approach based on earlier work by other scientists that incorporated a consideration of electron pairs in predicting three-dimensional structure. The valence shell is the outermost electron-occupied shell of an atom. The valence shell holds the electrons that are involved in bonding and are the electrons shown in a Lewis structure. The acronym VSEPR stands for the valence-shell electron pair repulsion model. The model states that electron pairs will repel each other such that the shape of the molecule will adjust, so that the valence electron-pairs stay as far apart from one another as possible. Molecules can be systematically classified according to the number of bonding pairs of electrons; as well as the number of nonbonding, or lone pairs, around the central atom. For the purposes of the VSEPR model, a double or triple bond is no different in terms of repulsion than a single bond. Summary • VSEPR theory allows more accurate predictions of molecular shape. Review 1. Who did the major work in developing the VSEPR theory? 2. Where are the electrons that are involved in bonding? 3. What is the basic idea behind the VSEPR theory?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.12%3A_Bond_Energy.txt
How does an electroscope work? An electroscope is a device used to study charge. When a positively charged object (the rod) nears the upper post, electrons flow to the top of the jar, leaving the two gold leaves positively charged. The leaves repel each other since both hold positive, like charges. The VSEPR theory says that electron pairs, also a set of like charges, will repel each other such that the shape of the molecule will adjust, so that the valence electron pairs stay as far apart from each other as possible. Central Atom with No Lone Pairs In order to easily understand the types of molecules possible, we will use a simple system to identify the parts of any molecule: • A = central atom in a molecule • B = atoms surrounding the central atom Subscripts after the B will denote the number of B atoms that are bonded to the central A atom. For example, AB$_4$ is a molecule with a central atom surrounded by four covalently bonded atoms. Again, it does not matter if those bonds are single, double, or triple bonds. AB$_2$: Beryllium Hydride $\left( \ce{BeH_2} \right)$ Beryllium hydride consists of a central beryllium atom with two single bonds to hydrogen atoms. Recall that it violates the octet rule. $\ce{H-Be-H}\nonumber$ According to the requirement that electron pairs maximize their distance from one another, the two bonding pairs in the $\ce{BeH_2}$ molecules will arrange themselves on directly opposite sides of the central $\ce{Be}$ atom. The resulting geometry is a linear molecule, shown in the figure below in a "ball and stick" model. The bond angle from $\ce{H-Be-H}$ is $180^\text{o}$ because of its linear geometry. Carbon dioxide is another example of a molecule which falls under the AB$_2$ category. Its Lewis structure consists of double bonds between the central carbon and the oxygen atoms (see figure below). The repulsion between the two groups of four electrons (two pairs) is no different than the repulsion of the two groups of two electrons (one pair) in the $\ce{BeH_2}$ molecule. Carbon dioxide is also linear (see figure below). AB$_3$: Boron Trifluoride $\left( \ce{BF_3} \right)$ Boron trifluoride consists of a central boron atom with three single bonds to fluorine atoms (see figure below). The boron atom also has an incomplete octet. The geometry of the $\ce{BF_3}$ molecule is called trigonal planar (figure $3$). The fluorine atoms are positioned at the vertices of an equilateral triangle. The $\ce{F-B-F}$ angle is $120^\text{o}$ and all four atoms lie in the same plane. AB$_4$: Methane $\left( \ce{CH_4} \right)$ Methane is an organic compound that is the primary component of natural gas. Its structure consists of a central carbon atom with four single bonds to hydrogen atoms (see figure below). In order to maximize their distance from one another, the four groups of bonding electrons do not lie in the same plane. Instead, each of the hydrogen atoms lie at the corners of a geometrical shape called a tetrahedron. The carbon atom is at the center of the tetrahedron. Each face of a tetrahedron is an equilateral triangle. The molecular geometry of the methane molecule is tetrahedral (see figure $5$). The $\ce{H-C-H}$ bond angles are $109.5^\text{o}$, which is larger than the $90^\text{o}$ that they would be if the molecule was planar. When drawing a structural formula for a molecule such as methane, it is advantageous to be able to indicate the three-dimensional character of its shape. The structural formula below is called a perspective drawing. The dotted line bond is to be visualized as receding into the page, while the solid triangle bond is to be visualized as coming out of the page. Summary • Electron pairs repel each other and influence bond angles and molecular shape. Review 1. What are the bond angles in carbon dioxide? 2. What molecule has bond angles of 109.5°? 3. What is the geometry of the BF3 molecule?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.14%3A_Molecular_Shapes-_No_Lone_Pairs_on_Central_Atoms.txt
How can all these clothes fit into such a small space? When we travel, we often take a lot more stuff than we need. Trying to fit it all into a suitcase can be a real challenge. We may have to repack or just squeeze it all in. Atoms often have to rearrange where the electrons are in order to create a more stable structure. Central Atom with One or More Lone Pairs The molecular geometries of molecules change when the central atom has one or more lone pairs of electrons. The total number of electron pairs, both bonding pairs and lone pairs, leads to what is called the electron domain geometry. When one or more of the bonding pairs of electrons is replaced with a lone pair, the molecular geometry (actual shape) of the molecule is altered. In keeping with the A and B symbols established in the previous section, we will use E to represent a lone pair on the central atom (A). A subscript will be used when there is more than one lone pair. Lone pairs on the surrounding atoms (B) do not affect the geometry. AB$_3$E: Ammonia, $\ce{NH_3}$ The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom (see figure below). The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with $\ce{CH_4}$. In the ammonia molecule, one of the electron pairs is a lone pair rather than a bonding pair. The molecular geometry of $\ce{NH_3}$ is called trigonal pyramidal (see figure below). Recall that the bond angle in the tetrahedral $\ce{CH_4}$ molecule is $109.5^\text{o}$. Again, the replacement of one of the bonded electron pairs with a lone pair compresses the angle slightly. The $\ce{H-N-H}$ angle is approximately $107^\text{o}$. AB$_2$E$_2$: Water, $\ce{H_2O}$ A water molecule consists of two bonding pairs and two lone pairs (see figure below). As for methane and ammonia, the domain geometry for a molecule with four electron pairs is tetrahedral. In the water molecule, two of the electron pairs are lone pairs rather than bonding pairs. The molecular geometry of the water molecule is bent. The $\ce{H-O-H}$ bond angle is $104.5^\text{o}$, which is smaller than the bond angle in $\ce{NH_3}$ (see figure below). AB$_4$E: Sulfur Tetrafluoride, $\ce{SF_4}$ The Lewis structure for $\ce{SF_4}$ contains four single bonds and a lone pair on the sulfur atom (see figure below). The sulfur atom has five electron groups around it, which corresponds to the trigonal bipyramidal domain geometry, as in $\ce{PCl_5}$ (see figure below). Recall that the trigonal bipyramidal geometry has three equatorial atoms and two axial atoms attached to the central atom. Because of the greater repulsion of a lone pair, it is one of the equatorial atoms that are replaced by a lone pair. The geometry of the molecule is called a distorted tetrahedron, or seesaw. Table $1$: Geometries in Which the Central Atom Has One or More Lone Pairs Total Number of Electron Pairs Number of Bonding Pairs Number of Lone Pairs Electron Domain Geometry Molecular Geometry Examples 3 2 1 Trigonal Planar Bent $\ce{O_3}$ 4 3 1 Tetrahedral Trigonal Pyramidal $\ce{NH_3}$ 4 2 2 Tetrahedral Bent $\ce{H_2O}$ 5 4 1 Trigonal Bipyramidal Distorted Tetrahedron (Seesaw) $\ce{SF_4}$ 5 3 2 Trigonal Bipyramidal T-shaped $\ce{ClF_3}$ 5 2 3 Trigonal Bipyramidal Linear $\ce{I_3^-}$ 6 5 1 Octahedral Square Pyramidal $\ce{BrF_5}$ 6 4 2 Octahedral Square Planar $\ce{XeF_4}$ Summary • The presence of lone pair electrons influences the three-dimensional shape of the molecule. Review 1. Why does water have a bent geometry? 2. Why is ammonia not a planar molecule? 3. How would we write the configuration for xenon tetrafluoride using the ABE system?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.15%3A_Molecular_Shapes_-_Lone_Pair%28s%29_on_Central_Atom.txt
What makes people share? Have you ever spent time with someone you really didn't like? You had nothing in common with them and did not want to have anything to do with them. On the other hand, there are people that you enjoy being with. You have a lot in common and like to share with them. Atoms work the same way. If there are strong differences in their attraction of electrons, one atom gets the electrons and the other atom loses them. If they are similar, they share electrons to form a covalent bond. Bond Polarity Electronegativity is defined as the ability of an atom to attract electrons when the atoms are in a compound. Electronegativities of elements are shown in the periodic table below. The degree to which a given bond is ionic or covalent is determined by calculating the difference in electronegativity between the two atoms involved in the bond. As an example, consider the bond that occurs between an atom of potassium and an atom of fluorine. Using the table, the difference in electronegativity is equal to $4.0 - 0.8 = 3.2$. Since the difference in electronegativity is relatively large, the bond between the two atoms is ionic. Since the fluorine atom has a much larger attraction for electrons than the potassium atom does, the valence electron from the potassium atom is completely transferred to the fluorine atom. The diagram below shows how difference in electronegativity relates to the ionic or covalent character of a chemical bond. Nonpolar Covalent Bonds A bond in which the electronegativity difference is less than 1.7 is considered to be mostly covalent in character. However, at this point we need to distinguish between two general types of covalent bonds. A nonpolar covalent bond is a covalent bond in which the bonding electrons are shared equally between the two atoms. In a nonpolar covalent bond, the distribution of electrical charge is balanced between the two atoms. The two chlorine atoms share the pair of electrons in the single covalent bond equally, and the electron density surrounding the $\ce{Cl_2}$ molecule is symmetrical. Note that molecules in which the electronegativity difference is very small (<0.4) are also considered nonpolar covalent. An example would be a bond between chlorine and bromine ($\Delta$EN $=3.0 - 2.8 = 0.2$). Polar Covalent Bonds A bond in which the electronegativity difference between the atoms is between 0.4 and 1.7 is called a polar covalent bond. A polar covalent bond is a covalent bond in which the atoms have an unequal attraction for electrons, and so the sharing is unequal. In a polar covalent bond, sometimes simply called a polar bond, the distribution of electrons around the molecule is no longer symmetrical. An easy way to illustrate the uneven electron distribution in a polar covalent bond is to use the Greek letter delta $\left( \delta \right)$. The atom with the greater electronegativity acquires a partial negative charge, while the atom with the lesser electronegativity acquires a partial positive charge. The delta symbol is used to indicate that the quantity of charge is less than one. A crossed arrow can also be used to indicate the direction of greater electron density. Simulation If it wasn't for our understanding of bond polarity, we'd have a really tough time cleaning our clothing. Use this simulation to understand how the unique polarity of soap molecules helps to clean our clothing! Summary • The electronegativity of an atom determines how strongly it attracts electrons to itself. • The polarity of a bond is affected by the electronegativity values of the two atoms involved in that bond. Review 1. If two atoms bonded together have an electronegativity difference of 1.9, what is the bond type? 2. What would be the bond type for BH2? 3. Your friend tells you that the LiF bond is covalent. Are they correct? Why or why not.	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.16%3A_Bond_Polarity.txt
How cold are ultracold polar molecules? Ultracold polar molecules have been created by research scientists. These "molecules" consist of $\ce{Rb}$ and $\ce{K}$ atoms excited by lasers to form a type of $\ce{Rb-K}$ compound where the $\ce{Rb}$ has a positive charge and the $\ce{K}$ has a negative charge. The material is formed at temperatures extremely close to absolute zero. The researchers believe these techniques will help them make new reactions and new materials. Polar Molecules A polar molecule is a molecule in which one end of the molecule is slightly positive, while the other end is slightly negative. A diatomic molecule that consists of a polar covalent bond, such as $\ce{HF}$, is a polar molecule. The two electrically charged regions on either end of the molecule are called poles, similar to a magnet having a north and a south pole. A molecule with two poles is called a dipole (see figure below). Hydrogen fluoride is a dipole. Polar molecules orient themselves in the presence of an electric field with the positive ends of the molecule being attracted to the negative plate, while the negative ends of the molecules are attracted to the positive plate (see figure below). For molecules with more than two atoms, the molecular geometry must also be taken into account when determining if the molecule is polar or nonpolar. The figure below shows a comparison between carbon dioxide and water. Carbon dioxide $\left( \ce{CO_2} \right)$ is a linear molecule. The oxygen atoms are more electronegative than the carbon atom, so there are two individual dipoles pointing outward from the $\ce{C}$ atom to each $\ce{O}$ atom. However, since the dipoles are of equal strength and are oriented this way, they cancel out, and the overall molecular polarity of $\ce{CO_2}$ is zero. Water is a bent molecule because of the two lone pairs on the central oxygen atom. The individual dipoles point from the $\ce{H}$ atoms toward the $\ce{O}$ atom. Because of the shape, the dipoles do not cancel each other out, and the water molecule is polar. In the figure below, the net dipole is shown in blue and points upward. Some other molecules are shown in the figure below. Notice that a tetrahedral molecule such as $\ce{CH_4}$ is nonpolar. However, if one of the peripheral $\ce{H}$ atoms is replaced with another atom that has a different electronegativity, the molecule becomes polar. A trigonal planar molecule $\left( \ce{BF_3} \right)$ may be nonpolar if all three peripheral atoms are the same, but a trigonal pyramidal molecule $\left( \ce{NH_3} \right)$ is polar. Simulations Why does water form droplets? How does soap get our clothes clean? Hint: both actions have to do with polarity. Try these simulations to learn more: How are stains removed from your clothing? Why do liquids form droplets? Summary • Polar molecules result from differences in the electronegativity of the atoms in the molecule. • Dipoles that are directly opposite one another cancel each other out. Review 1. Why is a polar molecule called a dipole? 2. The molecular geometry of NH3 is trigonal pyramidal. What about the molecular geometry of NH3 makes it a polar molecule? 3. What happens to polar molecules when an electric field is applied?	textbooks/chem/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/09%3A_Covalent_Bonding/9.17%3A_Polar_Molecules.txt

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